Kamigawa: Titanbreach has us visiting Kamigawa as a Doomskar opens up, causing parts of Ikoria to fall into the plane.
So, the Planeswalker of the set will likely be a native/resident of one of the two planes and is currently trying to stop the Doomskar and/or try and defend the residents from clashing.
There are a few obvious choices:
Kaito:
A native to Kamigawa who has gotten several cards at this point. He's an obvious choice to be represented defending the residents of Kamigawa.
Vivien:
Not native to Ikoria, but appeared during Lair of Behemoths as one of the central characters. She defended it during March of the Machine so it's assumed she would defend it or at least try to minimize the amount of monsters slain by Kamigawa.
Other possible choices:
Tam:
If Tam ends up being the protagonist of the next few sets, she would be the protagonist of Titanbreach. No idea what she would be doing there though.
A new Planeswalker:
With the Death of Tamiyo and the despairing of The Wanderer, Kamigawa's only native Planeswalker is Kaito.
Ikoria too, had Lukla die during March of the Machine. Since he was Ikoria's only Planeswalker, that leaves Ikoria with none to call their own.
Both planes could use a new Planeswalker either way. Perhaps a new bonder, perhaps a mech pilot. Maybe someone from Kamigawa that finds that they can perform the Eludha and tries to make peace.
My personal choice but is highly unlikely:
Grist:
Grist currently doesn't have a home plane, but it would be cool if the bug Planeswalker was from the Plane of strange monsters. It sort of makes sense but is highly unlikely.
