I've been meaning to bring up this topic for a while since Oath of the Gatewatch made its appearance. What can we expect from the prospect of other planeswalker Oaths into the Gatewatch? I encourage discussion of anything from what characters could conceivably join the Gatewatch, how and why, in what way they might contribute, or even simply the predictable Legendary Enchantment Oath cards associated.

For example: Liliana likely will join, just my opinion. It's a wonder why, but I do surmise that with Zendikar safe Jace may continue where he left off with accepting Liliana's request for help. Only thing I could guess is she agrees to join and help (when needed) if the Gatewatch can assist her with putting Razaketh & Belzenlok in their place (she likely will view it as if she's using the Gatewatch) but for a guess at her card--

Oath of Liliana
Legendary Enchantment (R)
When ~ enters the battlefield, put one 2/2 black Zombie creature tokens onto the battlefield.
Whenever you activate a loyalty ability of a planeswalker, target opponent loses 1 life and you gain 1 life.

Though I have more curiosity about Ugin and Karn. Ugin joining is 50/50 at this point, I can't see him as a primary member, but perhaps as a Zordon to Gideon-Jace-Chandra-Nissa's Power Rangers. Karn (or perhaps Koth) could very well join and draw the Gatewatch against New Phyrexia. But... the cards.

Oath of Ugin
Legendary Enchantment (R)
Devoid
When ~ or a planeswalker enters the battlefield under your control, ~ deals 1 damage to target creature or player.
Planeswalker spells you cast cost less to cast.

Oath of Ugin or
Legendary Enchantment
When ~ enters the battlefield, manifest the top card of your library.
Planeswalker spells you cast have "Morph ". Whenever a planeswalker you control is flipped face-up, put X loyalty counters on it.

Oath of Karn
Legendary Enchantment Artifact?
When ~ enters the battlefield, exile target permanent.
Creatures can't attack a planeswalker you control their controller pays for each of those creatures.
: Put a loyalty counter on target planeswalker.

Idk, just speculation.

There is absolutely no chance Liliana would join. She's entirely too self motivated and had way too many of get own problems to deal with. Until the chain veil and her demons are dead, she's going to want help and bugger anything that gets into her way. She wants Jace's help, but she's already demonstrated she doesn't give a damn otherwise. Zendikar can bleed and so can the rest of the multiverse.

It is as she has always acted.

Sorin, on the other hand, is legitimately likely.

Being on a team with those kids should be below his dignity. But if it means they make him friggin' mono black again to complete the cycle, and if it's the only chance of him ever doing anything useful again, I'd probably be cool with it. I guess if it happens at all, we'll probably get Oath of Sorin in Eldritch Moon.

Then again, I wonder why they made it clear that the Origins 5 will be a big deal going forward, but only 4/5 of them join the Gatewatch.
Then honestly Oath of Sorin would be the same as the Oath of Liliana concept, just replace one 2/2 black Zombie creature token with two 1/1 black Vampire creature tokens with lifelink.

See Lily's rbrilliant take on Tezzeret's Oath.

Ultimately, any planeswalker is liable to join because Creative doesn't care about such petty little concepts like "character" or "consistency", but I think Domri is the most likely candidate for a future member because he is a little kid and very impressionable.

He's also patently a little ****. I don't see him doing anything for the greater good, he doesn't have the maturity. He'd rather smash someone else's tower of blocks than build one himself.

Can you imagine him doing absolutely anything someone in authority told him to? They'd have to literally bully him into it since aggression is the only thing a gruul recognizes.

they didn't say they'd be a big deal, they said they would play a part in the near future and wanted to acquaint people with them. Don't make a mistake of correlation.

Indeed. Liliana has some serious issues that need settled and soon. She has a demon or two left to kill, and the Chain Veil is still affecting her in weird ways. not only that, she'll probably have face what she did to Garruk at some point.

If after all of that, she actually learns some sort of moral lesson, then maybe I could see her joining the Gatewatch. But I doubt it.

Tezzert: "My body is made from an old gate, and my heart, an old watch."

Not sure Domri joining would be as simple as that. There's a handful of planeswalkers (like Domri) whom we know little of their experience with multiplanar dangers (like Bolas, Eldrazi, New Phyrexia, etc) but for fun:

Oath of Domri
Legendary Enchantment (R)
When ~ enters the battlefield, target creature you control fights target creature your opponent controls.
At the beginning of your upkeep, look at the the top X cards of your library, where X is equal to the number of planeswalkers you control. You may put a creature card among them onto the battlefield. Then put the rest on the bottom of your library in any order.

We've seen Domri's ascension, we know he's got virtually no experience with anything.

From the Tublr thread:

Bolas: The multiverse is mine, what makes you think I ever stopped watching?
Narset: For the secrets yet hidden, I'll keep watch.
Vraska: For sins yet unpunished, I'll watch from the shadows.
Koth: For planes unburdened by my tragedy, I'll keep watch.
Tibalt: For screams yet unsung, I'll keep watch.
Tamiyo: In the name of the moon, I'll punish you.

Like I said, he's a kid. He'll probably think one of the Gatewatch is like so cool and join just by example.

Also, the flavour text in Predator's Rapport implies that he idolises the power of luv.

It won't be Jace or Gideon, so that leaves only Chandra or Nissa to stand in as his super sugoi onii-chan sempai.

Domri needs to have one of those metaphorical tower of blocks come crashing down on him. Then the owner of said tower needs to drag him out of the rubble and dish out tenfold whatever Domri was when he was kicking over those metaphorical towers. It would help him grow up quick. I don't see him as one of the Gatewatch regardless. Sorin's got a better chance of making the fifth member at this point.

Bots got smarter, we should keep watch.

Joiners will be inevtiable. MaRo made that obvious on Tumblr. Not having Lili join seems like a punt in terms of the direction Marketing is pushing everything. I imagine that they won't have others join until someone leaves however. The exception being Ajani, because white is the good guy color and he would make sense in the Avengers.