Hard to get a read on Joker since he's not actually playable for another 7 hours (give or take), but form looking at him, he seems like he'll have some very clear counters. I think pretty much every heavy can do well since Joker can't use Arsene until he's got some damage built up and that's playing a dangerous game against characters like, say, Ganondorf who can just Spartan Kick you off-screen at like 60%.
Incineroar might actually be a surprising counter to him, too. Incineroar's side B will just ignore Joker trying to use Rebellion Guard and his other throws are really strong too. Plus, Joker, before he can use Arsene, will likely just be taking pot shots via his gun (which he can 'Revenge') or use Eiha 'Which again can be Revenged) or camping and waiting for Arsene to become active.
Plus, with the continual damage of Eiha, Incineroar might actually be able to build continuous charge from the damage, allowing him to just super Lariat Joker and insta-kill him.
That reminds me, based on earlier data we had, what is Joker's weight class? Guess that'll help determine how easy a time he'll have too. If he's a heavyweight, then the other heavies would be as big a problem; same with medium. If he's lightweight, or even on the lighter side of 'medium'; his natural counter could easy be 'all heavies'.
