Since this post was made, Future Trunks, Krillin, Piccolo, Android 18 (with 17 as an assist), and Android 16 have all been announced as being on the base roster, along with Super Saiyan Blue Goku and SSB Vegeta as unlockable characters.
The game's website also hints that we'll be getting multiple forms for Gohan and Buu, since they (along with Goku and Vegeta) have additional details listed beside their names on their character pages.
I've done a mock-up of what I think the roster could be like, though I'm not 100% sure about some of the choices.
I figure there will be a lot of overlap between Dragon Ball Z: Extreme Butoden, the 3DS fighting game made by Arc System Works, and this game in terms of their rosters. The only character that I cut was Broly, since Kale is very similar, and I didn't include Jiren because I figure that there's not much known about him at the point where they're developing the game. (so far, in the ongoing arc, I'm pretty sure he's only attacked a grand total of once)
Aside from the roster of Extreme Butoden (minus Broly) and the already-announced characters, I included the following people:
Tien: the three major Dragon Ball heroes who weren't in Extreme Butoden were Tien, Yamcha, and Roshi. Yamcha has the least varied moveset of the three, and while Roshi nearly made the cut, Tien's more consistently prominent. (plus, he can fly, and one of the producers is a fan of him from what I've read)
Hit: of the opponents in the Universe 6 vs. Universe 7 arc, Hit is easily the most unique and popular. Botamo and Magetta may be unique, but they aren't all that noteworthy, and Frost and Cabba may be quite important, but they aren't unique.
Fused Zamasu: aside from being the most unambiguously evil villain that Super has, he seems like a good fit for the "ultra cheap final boss" concept that most fighting games run with. (either him or Beerus, but Zamasu has the look for it)
Kale: I went with her over Broly because, while both are popular, Kale's more recent and has more of a personality than "destroy Kakarot!"
Caulifla: it seems weird to have Kale and not include her as well; besides, of the Universe 6 Saiyans who have been shown, she seems to be the most prominent.