Thinking purely in terms of business strategy and not necessarily what I want to see, it makes the most sense that Shulk is the one to be revealed.
-It would most logically be a follow-up advertisement for something revealed at E3, to help build hype for an upcoming game (point in case: Rosalina and Little Mac didn't receive this treatment, and neither of them have new games on the horizon).
-Following that logic, the franchises not already represented in Smash were Xenoblade, Bayonetta and Splatoon (am I missing one?)
-Splatoon girl is the least likely of the three. Smash has always only included characters from existing franchises. While it's possible to break from that tradition, it doesn't make business sense - nobody knows or relates well enough to Splatoon girl yet to want to get hyped up over a 3 minute reveal. There is currently no fan base to cater to.
-Of the three, I place Bayonetta in the middle. She doesn't fit Smash's theme at all (but as Snake has shown, that's not a reason for exclusion), but more importantly her trailer was the only one to include some adult language. It's really hard to see this very adult sex symbol placed in Smash. However, we did see her wearing Nintendo character costumes - specifically, costumes matching popular Smash Bros. characters. Was that a hint at something? (I doubt it, but hey, who knows?) I'm unaware of the size of Bayonetta's fan base, so I can't comment on it.
-That leaves Xenoblade X, which was arguably one of the most impressive trailers in Nintendo's lineup, a recent first-party acquisition, and carries the most fervent fan base of the three. Shulk is also a rather unique character with an interesting weapon - yeah, I hate seeing yet another swordsman (potentially two more if Chrom is one the way as well), but the Monado has some interesting tricks that could translate into some fun gameplay. However, it is worth noting that Shulk was not in the new Xenoblade X trailer.
Considering the way they're breaking tradition with this reveal, and video games have always been the tradition, it's clear to me that it is most definitely Goku being revealed.