I want to make clear I do NOT think Goku is possible. It would be too polarizing to devote one of the very, very few remaining possible character slots to an anime character. It would be possible to do in theory, more so than Shrek anyway, but not right now, definitely not. That said, in the spirit of this thread, here's an up and coming Smash Youtuber who's reasonably big in the Youtube Smash scene, reacting to the recent direct. At the end, he (jokingly, I believe) discusses the possibility of Goku in Smash Ultimate. His argument, summarized, is that the trailers seem to imply that if there is a story mode, there will be heroes and villains, and that therefore the trailers we've seen so far are actually from the story mode, and that therefore, Luigi, Mario, and Megaman are canonically dead. If they're dead, they'll need someone to resurrect them. Enter Goku. Also, that punch between K.Rool and DK looked like a straight up DBZ scene, and he makes the comparison between such a scene.
Discussion of hero and villain theme and possibility of Goku begins at 17:00, comparison of DK-K.Rool punch and Goku-Vegeta punch is at 18:40.