Here's my ideas as I can throw them together now:
For the multiplayer, I think it might be cool to do the stories that way. So you could have a scene outta the story, with different people being different roles. You could end up as Goku with everything riding on your shoulders, or get a chance to see what life is like trying to make what difference you can with Krillin's limited powers but nice variety of techniques. People might be going "aww man" but I think it'd be cool to have a chance playing a weaker char in a definate role kind of situation. Is that just me?
Ultimately thought, there should for SURE be some multiplayer mode where everyone's balanced, or at least a large group of players are. And if there's going to be an ESF-style grouping of who's-who in DBZ, perhaps play up the differences between the characters. Give each one a slightly different playing style.
And make sure there's something in there so people who haven't played DBZ can follow it!!!
Oh, and maybe have a city level where you can fly around and into skyscrapers. Blasting your way into and through actual buildings would rock! Although if your shots are 3-dimensionally large (like in ESF where they're large objects which can brush against solids and explode) I guess there'd need to be a way where you could restrict Kamehameha's so they wouldn't blow up on the ceiling & floor.
... well it's late, maybe give those ideas some thought.