I mean maybe? I think the only real rules was that the character originates from a game. There was I think another ACTUAL one was "showing up on a Nintendo console at some point" Cloud didn't have a ton of Nintendo appearances, but some Kingdom Hearts and 3DS Final Fantasy games apparently mattered enough when it came to adding him. Pretty much every character had done so that was added to Smash, so that would be the actual hurdle I can think of.
The main thing is that we don't know how nice Sony is willing to play. But they did make Lego Horizons be put on the Switch. I guess you could make a case for Aloy then.
Personally, if there's anyone form the Playstation side of things being added to Smash, I would go for Ratchet and Clank first, or Kratos. Not that I'd mind Sly or Jak or Kat and so on. There's other PS icons I'd want more, but they were never really owned by Sony directly, namely Lara Croft, Spyro and Crash. And for Western games as a whole, Scorpion and Sub-Zero.
Given how poorly the X-Box is doing this gen, I wonder if they'll play ball with Nintendo more now than they already have been. I mostly want to see more Killer Instinct stuff generally, but hey, it is what it is.