I'm just going to do this once to get it out of my system. Ideal/dream roster, with a clear focus on Super (particularly Universe 6) and cutting the fat when it comes with Z characters (only stuff you build a lot of fights around for single player). 30 characters, which may be asking too much, but is basically what I would hope for in a team fighter and may come to expect if you want to throw in DLC and/or a revision (more with a full on sequel).
Saiyans:
Goku
Gohan (Young)
Gohan (Adult)
Vegeta
Trunks (Future/Goku Black)
Gotenks*
Vegito
Other Heroes/Allies:
Krillin
Piccolo
Tien Shinhan*
Master Roshi OR Jaco OR Hercule*
Villains (and Reformed Villains):
Frieza
Android 17*
Android 18
Cell
Dabura*
Buu
Super Buu OR Kid Buu
Super:
Beerus
Tagoma(Ginyu) OR Champa OR Whis*
Zamasu*
Goku Black
Fused Zamasu
Universe 6:
Hit
Frost
Auta Magetta
Botamo*
Cabba
Caulifla
Kale
*-Slightly unsure about these, would be first to cut or swapped out. (Caulifa and Kale are a bit underdeveloped, but stand out character design wise, like a lot of Universe 6).
I don't see much value in filler or movie characters (Janemba would be fun though). I would love DB characters (Kid Goku, Tao, Arale, maybe even a shared Grandpa Gohan/Jackie Chun spot), but my bias is titled towards a focus on Super to make the game feel fresh. There have been so many games covering up to the Buu saga, even just considering 2D fighters. A lot of my logic is based around the admittedly unfounded idea that there is going to be a lot of focus on a single-player campaign, one that I'm, again, hoping shows favoritism toward Super.
There's four (and a half?) arcs they could go through: Battle of Gods, Resurrection F, Universe 6, and Goku Black (plus some more Universe 6 stuff with Hit and the other Saiyans, setting up Tournament of Power for a sequel). Fused Zamasu would make a great final boss, if not a Hit rematch or Goku vs Gohan. Someone like Tagoma may seem pretty forgettable compared to all sorts of characters you could pull from the series or movies, but he ends up playing a big role in Resurrection (anime), so you can get a bunch of single-player fights (including Frieza's training) out of him in an single-player version of that arc (plus a power-up where frog-Ginyu steals his body would be pretty fun); on the other hand, he isn't important at all (compared to Champa or Whis) and may not be the best possible choice in terms of the multiplayer (I lean on the idea that ASW are talented enough to strike a balance though). Obviously, if they can't make Tagoma a fun character, then dump him and just focus on what would make a great game.
So then, the Z material could be used as flashbacks to establish the characters who are relevant in the Super era (the ten participating in the ToP, more or less) in the form of highly abridged versions of those stories. For example, this is why I would prefer Super Buu over Kid Buu, because the former helps establish Gotenks, Adult Gohan, and Vegito, while the characters who fight Kid Buu are not lacking for screen time (fat Buu as an enemy). Saiyan/Namek would be fairly anemic, but I think with this set of characters (3 or so villains for Cell/Trunks's future and Buu), your bases would be covered (adding characters like Nappa, Android 19-20, etc. would feel like a bit of a waste even if they would be needed for a complete story).
While giving it some thought, I realized that a 3v3 fighter is a terrible format for a Dragon Ball campaign. Most fights are 1v1 and with plenty of 2v1, but it's not often for it to go beyond that. I have to imagine the single player will not make the most out of the 3v3 mechanics without either a lot of abstraction or divergence from the plot (e.g., you fight multiple Cells at once or Frieza fights along with him, acknowledged or otherwise). This wouldn't really be a bad thing I think.
Alright, with that, I'm retiring from dragon ball videogame fanfiction. Won't really be upset with who gets in as long as the game is fun. Fuck Broly if he gets in before Kale though.
EDIT: As for stages, I just want one that has a bunch of Frieza's troops in the background (Resurrection F) and any deflected shots blow them out of the sky.