If i have to pick a card:
Men With No King.
While there is no anti-steal tech in the game, the perma-steal cards are powerful, adn this one might be the most powerful since it has no restriction on who to steal. The new Reek can be pretty bad, as can Dubious Loyalties.
But I'm on the anti-ban side with Ktom and Krew.
the most likely card to
actually
be banned is probably Val or Blood of the First Men. I doubt either will be, but they just seem like the most realistic candidates. If Narrow Escape isn't banned then my guess is that Paper Shield or something very similar is coming back; although, I wish we had it now