So this got spoiled:

And I saw at least one “So is everyone going to join the Gatewatch now?” complaint.  Which seems fair.  If we have one of these every set or every block, I could see that getting old and it makes the stable of protagonists a bit large and unwieldy.

But I don’t think that’s going to happen.  And, while I’m no Flavor Judge, I ran through a quick checklist of other planeswalkers we know of (with help from @nostrangenames) and whether they’d be interested:

Other Planeswalkers reaction to the Gatewatch:

Venser – Dead
Xenagos – Also Dead
Elspeth – Mostly Dead
Tezzeret – Kinda Evil
Garruk – Probably Evil
Nicol Bolas – Super Evil
Ugin – Was kind of a jerk last time we met him, so probably no.
Kiora – Already bailed once
Tamiyo – Already said no
Narset – Too busy studying
Koth – Too busy pit fighting Phyrexians
Ashiok – Not really into making other people happy
Kaya – “This isn’t ghost murder”
Deretti – Too iconoclastic
Dack Fayden – Not enough phat lewt
Sarkhan – Living the Dragon life, so no
Arlinn – Living the Werewolf life, so also no
Ral Zarek – Maybe?
Vraska – Yeah.  This seems like a great idea.  Go for it.
Nahiri – Deserves a break and probably wouldn’t be invited anyway.
Sorin – Hahahaha.  No.
Tibalt – GO HOME TIBALT.  NO ONE LIKES YOU.

So… maybe Sparky, assuming there’s some good story reason for him to show up somewhere that isn’t Ravnica?  I would look forward to the Oath of Ral Zarek card which would, based on my experience,  be something like, “Whenever you cast a planeswalker spell, flip a coin.  If you lose the flip, look sad.  If you win the flip, IT DOESN’T MATTER BECAUSE YOU ARE NEVER WINNING THE FLIP.”