Perhaps, although they are really 4th party, but I get your point. Well, if it's not because of Sakurai's statement about them, it if just boiled down to "being a part of Nintendo's history" than "originally a video game character" than Goku's chances wouldn't be as unlikely as you suggest as there's been countless of DBZ games on Nintendo's consoles, even as far back as the Gameboy days (I should know as I had one game before). And with Namco have helped make the game, and Bandai is apart of Namco, they could do that just from principles of them having the possibility of doing so, if Sakurai didn't mind the idea.
But again, it's not something I'd expect to happen, but if somehow it does, I'd wouldn't care about it. Sakurai does whatever he does wants regardless, otherwise Ryu wouldn't be in this game.