Ah, okay, got it. I had completely forgotten there wasn't a Core Set every year before M10. And if we use Lorwyn block to introduce 'walkers that aren't the original Lorwyn Five (and are somehow connected to Lorwyn's story) and then have the Magic Origins/M10 Core Set with Jace, Chandra, Liliana, Garruk and Gideon after Lorwyn block? (I'm pushing out Ajani here because Gideon is in Chandra's novel and Ajani shows up in Alara anyway). Not going to force the Core Set idea too much, I just think it would be neat to have a better and earlier version of Magic Origins that ties in with the novels and comics...

If you did M10 early, it would be M09

The walkers should debut in Time Spiral. It just makes sense to have walkers show up alongside the events that redefined them. Heck, make the middle set of time spiral be the part where stuff gets fixed, and the last block shows how the world's changed.
For the debut, I also think that every colour should get one walker.

No how do we get all our walkers together?
I propose that the force of the Mending pulled nearby walkers out of the aether and into Dominaria. They're all like "Wha?" trying to figure stuff out. Since they're curious about how the sparks still working they start looking out ascending walkers. That accounts for everybody, but doesn't bind them together too tightly. They don't even all have to meet up.
Alternatively, a villain wants to undo the mending by sacrificing walkers to reopen the rifts. It doesn't even have to be a feasible plan; they just have to be willing to kill people in the attempt. Then they lure most of the rest of the cast there.
Or we could do like WOTC in Lorwyn and go "whatevs, here's some walkers".

I'd take Ajani as the first white walker here. Nothing pulled too strongly between him and Elspeth, but a somebody a little less human was dictated by my other choices.
Venser is the obvious choice for blue. He was already a character in the block. I would like to see a character arc about him maturing though, maybe even have it mirror the development of the franchise as a whole?
Sorin v Ravi is a tough choice. Despite WOTC's latest efforts I still like Sorin. Ravi has awesome potential though.
I'd definitely take Jaya over Chandra. Chandra can show up later maybe. I don't care.
Nissa should be saved for Lorwyn where she has a good reason to be. Garruk can show up in Time Spiral.

I would also kill the whole bit about accelerated aging catching up to the walkers post mending. I say they're just stuck in the bodies they had during the mending and age from there. Nicol Bolas' new problem is that his old walker body just isn't sustainable any more. the FTV art shows he's big enough to swallow other dragons he can't sustain all that without walker power. Those millennia of memories are now crammed into a squishy organic brain. The power he wielded can't be handled by any merely mortal form.

Two squishy brains. He has a secondary nerve cluster that functions like a brain.
But all the same, I'm firmly against including Ajani before his origin story in Alara.

That's a good point. (the second one. The first, while interesting, was not impactful)
If Elspeth is our white walker I guess that's not too bad for cast diversity. We'd have 2 human females (not counting Ravi), 2 human males (not counting Sorin) and 1 elf female. Pretty heavy on the heavily humanoid, but short of inventing characters we don't have too many options on that one.

Yeah, the vastly non humanoid is.... Very very limited. We can blame "market research" for that, since apparently the further from the strictly human model you get, the less connectivity the characters garner. (which is severe bs)

The more I think about it, the more I feel that another block between Time Spiral block and Lorwyn block might be a really good idea. I'm probably interfering with the original plan a bit too much, and I definitely understand why Yxoque doesn't want to start with too big a change. But since the time immediately after Time Spiral block is where things went wrong with the transition to the post-Mending storyline, we should probably consider all options before settling for a solution. Here are a few reasons why I think a completely different block after Future Sight would be a good option to introduce the new 'walkers:

- I think introducing planeswalkers in Time Spiral block isn't really optimal for the reasons AzureShade stated. It was weird and crowded enough as it was (not necessarily in a bad way, though). Plus, it would raise issues with the timeline and force us to compress the time between Future Sight and Alara to a few years at most. Unless we only use former Oldwalkers that either have nothing to do with the post-Mending storyline (then why bother?) or that would force us to rewrite huge portions of the later storyline to integrate them (very messy).
- Integrating a cast of five mono-coloured 'walkers into Lorwyn in a way that makes them both relevant for the story and still has them be icons of their respective colour is really hard. Plus, between all the changes and other weird things, Lorwyn was a bit too weird for most players at that time. It's probably better to start with a more down to earth setting that's interesting enough in its own right but doesn't draw unwanted attention at the expense of the 'walkers.
- Introducing 'walkers in a Core Set as I suggested earlier still leaves us with the problem of what to do with Lorwyn. Ideally, we'd want the mono-coloured baseline characters to be the first 'walkers that are introduced, rather than introducing story relevant but less recognisable characters in Lorwyn first.
- We're aiming for a tight connection between the new 'walker cards and their respective characters and stories, and going with what's in AoA, TPF and the webcomics seems like a great way of doing that (again, assuming that material was planned earlier and can be published right after Future Sight).

So, if we were to do a different block after Future Sight indeed, how about using Kephalai as a setting? Yeah, I know, that might seem like an odd choice at first glance, but hear me out. Chandra is shown doing stuff on Kephalai in both the webcomics and her novel, and both Jace and Gideon are there, too, at different points in time, because they were sent after her. Garruk could just do Garruk-stuff somewhere in the wilderness, we have to show more places than just Aretopolis after all. There's that one Savor the Flavor or Taste the Magic story that appears to be set on Kephalai and already mentions a couple of places, so we could go with those. That leaves us with Liliana, but I think the solution is simple. What if Kothoped's lair was on Kephalai and that was where Liliana blew him up with the Chain Veil? The plane seems to operate on vaguely Greek naming conventions anyway ('Kephalai' is derived from the Greek word for head, 'Aretopolis' means City of Virtue, and 'Kothoped' definitely sounds Greek, though I'm not sure if it actually means anything). Add that to the fact that we have no idea where Kothoped was from anyway, and we don't know what demons on Kephalai usually look like. Showing Kothoped's hiding place would give us another opportunity to show things outside of Aretopolis (not that we wouldn't do that in a three-set block anyway...). We could already allude to Bolas pulling the strings behind Liliana getting the Veil, which we know to be the case anyway. Kothoped sending Lili to get it only to be killed by her as a result always seemed stupid to me, so maybe he was manipulated by Bolas. This could be shown by including one of those cute mini-me imps Bolas sends to various planes in the comics. The cards could show events, people and places on Kephalai from the novels and comics and maybe even hint at The Infinite Consortium, Tezzeret, The Order of Heliud etc. in flavour text. This could also be where Jace meets Garruk for the first time, explaining why they know each other in the comics that are set after AoA.

Again, sorry to mess with the OP's premise, but I think the idea I proposed is at least worth considering. The start is probably the most important thing to get right.

If we're going to start this experiment in the Time Spiral Block (which I advise against), wouldn't it just be easier to introduce 'Walkers in the block as cards, then let the story chips fall where they may, then pick up the pieces after?

To-wit:
-Venser is released as our White 'Walker. His teleportation flicker magic is in White's pie as is some artifact magic. It won't mater that he's our White representative for long because the next time we see him he'll be White/Blue (and then be dead).
-Teferi is our Blue 'Walker. Teferi is iconic, old, has ton's of Blue abilities to draw from, and will eventually be de-sparked by the end of the story. It will however give players an awesome Blue 'Walker to play with until we get around to replacing him in later blocks.
-Leshrac is our Black 'Walker. Again, old character for the Vorthos to enjoy, dies at the end of the story, clearing space for Liliana in later sets.
-Jeska as our Red 'Walker. Again, Vorthos happy to see the end of that storyline and losing her gives us room for Chandra later.
-Freyalise is the Green 'Walker. Like most of the others, she too will die off, but be replaced by Nissa.

In the end you start out of the gate with four Old 'Walker cards to help storyline people transition into the idea that Planeswalker cards are going to be a thing moving forward. On top of that, none of them get in the way of future stories because they are dead or de-powered before things get too entangled. You can now time-jump to Lorwyn and start sprinkling in the new age of Planeswalkers.

I tend to connect with nonhumans far more than humans, so that could totally be a nice way to make people with social disorders feel more welcome. Also it makes for more interesting stories.

Also needs more tibalt.

Azure's idea for Time Spiral seems doable and very organic, yeah. Venser being white is pretty meh, but then again, if his creature card is white, too, I guess it will be slightly less awkward. Or we could consider making them all ally coloured, or at least not necessarily mono-coloured. That would still leave us with the question of how to introduce the other new 'walkers after that, though, so squeezing in another block before Lorwyn could still be an option .

Nah, I'd just time-jump to Lorwyn and then filter them in naturally. When we get to Scars of Mirrodin and Venser shows up again, he should look much older or explain that his druggie addiction of Blink-Moth Serum was some sort of youth treatment that sorta went off the rails on him. But then he'll die so it won't matter.

Basically, just filter in five mono-color 'Walkers into the four Lorwyn blocks. They can hang out with their appropriate people (Nissa/Elves, Chandra/Flamekin, Jace/Liliana/Faeries, Ajani/Gideon/Elspeth/Kithkin/Giants/Whatever). Really the hard part is nailing down who to have the white intro 'Walker be. People don't want to use Ajani because there is a younger form of him on Alara, Gideon makes the most sense next because otherwise we end up with a 4-1 female to male split and people start making Jace Harem jokes, and Elspeth really doesn't make much sense on the plane (also we don't know about Nahiri yet).

In fairness, Gideon doesn't mage any sense on the plane either. None of the humans do.

I don't know, I can easily see Chandra as a frequent visitor and friend to some flamekin. Jace being there to investigate Oona makes sense (because Jace is a meddling kid), and Liliana being there but not wanting to be would be fun, especially after the swap over to Shadowmoor where she might find things more to her liking...

But yeah, there's really no reason for any of the White 'Walkers to be there, but cycles are made to be filled and planeswalkers are unabashed tourists.

The problem is, there are no humans. It's not an issue of if the Walkers can find something to do there, but how the natives react to it. (poorly btw) If the Walkers are to show up, they basically can't take part unless hidden or disguised. The fact origins didn't have the elves trying to blatantly murder Nissa for not having horns was remarkably ignorant at best and downright insulting at worst. (I mean, drop in the ocean, but...)

Eh, Chandra can pass as an odd-looking flamekin...sorta, and Jace has illusion magic. Liliana would probably just kill whatever she runs across that looks at her funny.

But yes, some characters on Lorwyn would have to be a bit more knowledgeable about 'Walkers for them to make sense there. Which, since we're contemplating going so far as a write in a whole new block at this point....seems like the simpler solution?

EDIT: I mean does Tamiyo have to run from peasants with pitchforks all over Innistrad too?

Other than that, I'd vote we just skip Lorwyn all together and pick up things on Alara. We can filter in 6 'Walkers over 3 sets in the block (5 plus Bolas).

The only problem I predict with skipping Lorwyn is the complexity of Alara next to time spiral. Shadowmoor's color theme was meant to act as a complexity buffer, which is about the only success it had. But yeah, if we replace Sarkhan Vol with Sarkhan Dragonspeaker we can work with him in red and figure out something to do with blue since Tezzeret wasn't involved. Maybe Jace and Liliana for black and blue since they were tied into Grixis at this time.
About the only problem we encounter is solving for green and having too many white Walkers involved. I'd drop Elspeth since Ajani is much more important.

We still have the issue of introducing Tezzeret and Elspeth at some point.

Well since Tezzeret is from the Esper shard, I don't see why he couldn't fill in the Blue slot. Also, I don't suggest we cut the events of Lorwyn from the storyline or anything. Just that we follow Wizards' original plan of not putting out 'Walkers all the time. Of course this plan went straight out the window once they saw how popular 'Walker cards were, but we could take a break during Lorwyn pick things up in Alara.

EDIT: The big issue is that the story of Alara is about Bolas eating it and Ajani stopping him. With Red, Blue, and Black 'Walkers as ostensibly "servants of Bolas" (Sarkhan, Tezz, and Liliana), who would make sense as our Green 'Walker, or does it matter at all because Ajani is the one doing all the everything.

Lorwyn's events are entirely self contained. They have zero effect on the greater story if we dropped them. Not having Walkers appear there would Jace been entirely ok. In fact, we'd have had much less speculation that Maralen was secretly Liliana.

And Tezzeret hadn't been on Esper for approximately a decade when the conflux happened. It's always been an annoyance of mine since he was literally the only walker not involved with the story.

Edit: I also really really hate the entire "Liliana, servant of Bolas" thing. She was always the one going to get his help, not the other way round. She's a customer, not a servant.

I know Lilly doesn't consider herself a servant of Bolas, but he set up her contracts with the demons (and holds that over her when she wants his help wriggling out of them), he's rearranged her mind to protect her from Jace, and he always seems to be the person she runs to when she needs something done (much less so now that she has the Chain Veil and Jace). That's about as "customer" one can be as a dependent to their drug dealer. She may not be happy about it, she's done things because he told her to before.

Not really. Manipulating Jace was her idea and she's always the one who approaches him offering a deal, never vice versa. He's offered to take a more active role and she's told him to go shove it up his tail.

He may have more leverage in the relationship, but he's clearly fine ignoring her. More, every time they do that sort of thing, it damages Liliana's independence.

One thing I may do tomorrow (unless someone else beats me to it) is go through every 'Walker we have a card for and list when, in the story it is imperative that they show up. That way we know who we can and can't use to fill in holes and adjust things around.