So I guess the real question is, would Nintendo be willing to approach Sony? I remember someone said at least one of them still held a grudge against the other, but I can't remember which.
I just hope that if they DO decide to collab, it's with a character who would actually be interesting in Smash. Someone like Ratchet & Clank, Sly Cooper, or Kat from Gravity Rush. Just... no overly-realistic characters, okay? I know those are what Sony is known for these days, but... no.
Sly Cooper or Kat wouldn't happen at all if by some miracle we get a Playstation character.
Ratchet & Clank? Maybe, but I kinda still doubt it even if I'd love it.
Also, I guess you're talking about the likes of Nathan Drake, Ellie, and more. I don't think Nintendo would pick those. Maybe Nathan. But I mean... I'm sure they would be interesting in Smash, even if you can't picture it.
Then there's Kratos who has a crazy moveset potential and who is probably likely than all of those combined except for Ratchet. The only hurdle against Kratos is that God of War is not popular in Japan, but the same goes for Master Chief and Halo and people believes Chief is the next in line for a Microsoft rep.
About your first question, I don't think Nintendo is going to approach Sony. Keep in mind that the bad blood between the two is probably no longer a thing because most of the key figures, if not all, of both companies back in the day are not longer working there or at least not in important positions. But that doesn't mean they're going to work together, so I don't know. I guess it would take some time for Nintendo to go like "hmm, what if we talk with Sony for a Smash character?". If you ask me, I don't think Sony would be completely against the idea, but I guess they would be kinda hard to work with or with very expensive demands in exchange of their characters in that scenario.
To be honest, I'd not be licensing my IPs for cheap as well in that scenario lol