Modern Horizons spoilers started earlier this week. While I don't think we're getting any story with it, there is some juicy worldbuilding in the cards.
-Wherever Blade of the Sixth Pride is from is also home to some wonky purple goo changelings.
-Slivers are back and appear to have made it to Alara somehow.
-The card Planebound Accomplice appears to depict our second unknown planeswalker in the backround (as an aside, is that design sufficiently weird Tevish?)

Half a dude wired into a tree mech with his chest glowing incandescently?

Yeah, I dig it. That's the kind of stuff I've missed.

I'm not entirely sure WHAT is going on it that art other than what appears to be a glowing ghost lady in front of an Eldrazi.

The bit with the planeswalker sacrifice mechanic kinda goes both ways.

Let us formally welcome our newest planeswalker, Wrenn and Six!
According to the preview article "Wrenn is the being in front and Six is a Treefolk host carrying Wrenn around. Six is the sixth Treefolk, a mech-like vehicle for Wrenn to symbiotically pilot."

Arboreal cyborgs? Noice!

Kind of impressive. They went really weird with this and yet the curse of green red still holds true.
I don't like it.

Mechanically he's got some weird tension in that to make his +1 valuable you either want his emblem or want to not need it. He doesn't go off at once with Doubling Season either so that limits abuse some.

Conceptually I like that I called "Tree mech" when the article ended up calling his treefolk a mech. It means they did something right with the visuals.

Not sure how I feel in whole, but I at least appreciate the weirdness.

@Barinellos: What is the curse of green red? Just the general abundance of walkers in that slot?

Essentially, and how they really don't interest me at all personally.
Out of all them, the only that's done anything whatsoever to pique my attention was Xenogos.

They made Yawgmoth. I'm so happy.

I consider part of the problem to be that a lot of them are like "Make a creature slightly bigger, give it haste and trample or something" and it is not really clear what kind of magic they use other than liking creatures. In that respect this land focused walker is something new.

It also doesn't help that they suck out loud if you aren't already sitting pretty nicely on a board state already.

This.

And I like that the flavour of the Snow cards in the set isn't totally jarring next to the ones from Coldsnap, some of them even directly reference Dominaria's Ice Age. The new Snow basics look kinda alien and weird and I'll definitely stick to playing with the older versions (full art lands never really did much for me anyway), but apart from those, they wouldn't exactly stand out like a sore thumb in any of my Snow decks. Snow is definitely my single favourite mechanic that they brought back for this set by a long shot.

The funny thing is, I almost made a wishfully-ironic post in the "Where do we go from here?" thread a while ago, boldly stating that we were obviously in for a Snow-heavy set called Dominaria: Northwind that would take us to the Northland, where the Weatherlight crew uncovers a secret conspiracy of Jodah's old enemy Zur the Enchanter who has succeeded at making himself the immortal king of the Northland and is trying to take over Terisiare. Something something, Marit Lage... (Seriously, can you imagine a reprint of Into the North with artwork of the Weatherlight flying towards a sky full of polar lights?) And yes, this is totally going to happen in a future Dominaria D&D adventure of mine if we ever get that far.

Some day I'll have to make a thread called "Why the Northland is secretly awesome" or something. So much potential...

Wait, and now they actually printed Marit Lage's Slumber??? Dang, looks like I really missed an opportunity to accidentally predict the future

On an unrelated note, did you notice that there are a big and a small moon in the artwork of Morophon the Boundless? And that there's a new artefact shapeshifter with flavour text by Tocasia? What I'm getting at is, are the new shapeshifters all from Dominaria? If yes, does that mean Blade of the Sixth Pride also is? Like, maybe that gives us a glimpse of modern day cat people in Jamuraa? On the other hand, I believe none of the existing tribes looked like lion people, right? Maybe I'm reading way too much into this... But if Morophon is indeed from Dominaria, do the bears and the snowy landscape in the background mean that all of the bear and Snow related cards in this set are also supposed to be from there? *puts on tinfoil hat* All right, I guess my brain is just desperate to fill in blanks in Dominaria's worldbuilding wherever possible because we only got one set when we returned there...

No, its from Mewni.

Fountain of Ichor seems to be set on Ixalan. Is the titular "ichor' Phyrexian oil? How likely am I right, how likely is it to actually reflect something WOTC's considering, and what do they stand to gain by Phyrexianizing Ixalan in particular?

Robot Zombie Dinosaurs

MaRo said it is not. Take that how you will.

I think "ichor" is just a fantasy word for "oil" here and it's just a fossil fuel joke.

I could see that, though it's not a word for oil. It's more akin to blood in the traditional usage.