If we divide things up by arc, we can see that we actually have room for a decent amount of representation on the villains' side:
Saiyan Arc
Raditz (likely to get cut)
Nappa (flip of a coin on this one)
Vegeta
Namek Arc
Ginyu (also a coin-flip)
Freeza
Android Arc
16 (most likely to get cut IMO)
17 (coin-flip)
18
(19 & 20 are never making it in - they're not even in XV2, they're not gonna be in DBF)
Cell Arc
Perfect Cell and only Perfect Cell (sorry, not enough people care for his imperfect or semiperfect forms)
Buu Arc
Fat Buu
Super Buu
Kid Buu (either Kid Buu or Super Buu will probably not make the cut)
Which just leaves the Z-fighters themselves. I strongly doubt they'd bother giving them variations for each arc - Goku, Gohan, and Vegeta are clearly using their Cell saga variations, and at most I'd expect maybe an adult form for Gohan (using Potential Unleashed) and a SSB form for Goku and Vegeta. That still leaves a significant chunk of the cast unaccounted for, assuming 20ish characters in the final game (excluding DLC ofc).
I expect Beerus is a shoo-in. So is Piccolo. Krillin is too important to cut. Future Trunks is super-popular. Tien is a coin-flip. Yamcha is unlikely (sorry, buddy, but the way Super has treated you has made it absolutely clear where they stand on this). Chiaotzu isn't even in XV2, you can forget about him. Goten and Trunks could be in... but if they are, it's probably as Gotenks. Vegetto/Vegerot is a definite shoo-in.
I'm going to completely discount movie characters because the roster is likely to be tightly focused for the initial release, and judging by the Z on the logo, mostly tightly focused on Z fighters.
In light of these considerations, my guess for the final roster looks like this:
Goku (Cell Saga)
SSB Goku (Resurrection of F)
Vegeta (Cell Saga)
SSB Vegeta (Resurrection of F)
Gohan (Cell Saga)
Ultimate Gohan (Buu Saga)
Krillin (Namek/Cell Saga)
Piccolo (Android Saga)
Future Trunks (Android Saga)
Gotenks
Vegetto/Vegerot
Nappa
Captain Ginyu
Freeza (Resurrection of F)
18
Perfect Cell
Fat Buu
Super Buu or Kid Buu
Lord Beerus
The last slot would go to one of the coinflip characters. This is, of course, assuming that Goku & Vegeta's SSB variations count as separate slots, and that they make the cut in the first place (who knows - they might not). We'll see how things go.