Do you guys think that nintendo will ever add a pc exclusive character to the roster?
Or really something like a playstation exclusive or xbox exclusive
Depends if you mean exclusive as in the franchise or the physical character. Franchise exclusive, probably not. That's advertising 100% for another system(while they did advertise Banjo-Kazooie on Microsoft, it also had the caveat of being a Nintendo-owned character at one point and the NSO for N64 might've been been in the plans by then, meaning they weren't going to really lose out on it).
Just a character, well, most aren't exclusive as much as they used to be. Some aren't even a cameo anymore(Master Chief is a cameo in Minecraft. In Fortnite, he's part of the official storyline, making him a proper appearance moreso). But see the above of why it might not matter who the character is alone. The fact of the matter is, the reason why "having a Nintendo appearance" is a thing is more that any character is an advertisement. Strictly advertising for a rival company isn't exactly something most companies would want to do. Albeit, there's also licensing; if a character had a previous Nintendo appearance, no matter how small, this means the companies can cooperate again. No matter what the appearance is, this is an official character appearance at the end of the day and requires licensing and agreements for it.
That, and the franchise is probably way more important than the character. While Cloud had a very minor appearance(he was in spin-offs and had no major roles even then) before getting into Smash, Sakurai only at most said "he isn't sure what he would've done", making it pretty clear he hasn't dealt with the particular situation. Nintendo on the other hand, well, if the franchise itself has not been on their consoles, would require licensing and agreements to figure out how to deal with these things. Stuff like Fortnite being advertised this way "check out how this character appears in this game" is a good loophole in itself to avoid only advertising a rival company, that might not fly for anyone else. Fortnite's creators also have to agree to this, and Microsoft gets the short end of the stick if we go with the Master Chief example. Thus, it's highly complicated.
Nintendo isn't a purism crazy company in the sense of Smash. They directly chose Joker, who isn't even based upon his Nintendo spin-off appearance of Q2 in any way, so they already know that it's no big deal. Persona has been on Nintendo before, or at least was planned enough, that the franchise isn't "Nintendo-free" either, which may also make it easier to justify the addition from their end. Besides that, let's also note the caveat that they wanted to work heavily with Atlus to bring over tons of content, so this all leads up to tons of useful agreements that benefit all. You can't always get this with every franchise.
Hence, it's a complicated mess and there's no way to tell whether having a Nintendo appearance is even that key of a factor so much as the franchise itself. Also, keep in mind for the above example is that Persona and Shin Megami Tensei is treated as separate franchises, all the way through Smash, so SMT being on Nintendo in itself didn't matter. Atlus working with Nintendo multiple times in the past is actually important. That, and it could also depend on the factor of who owns a particular character and why they're exclusive to a console or PC. Those companies might be under special agreements, so a Smash appearance could be downright impossible only for that sole reason.
But yep, complicated.