They need to do what
Injustice is doing: not only a great solo story mode with lots of cutscenes, but also many entertaining (animated and multi-leveled?) stages covering pretty much every kind of place shown in the story and movies, plus a hundred costumes so that all costumes ever shown on the comics (or manga in that case) are available.
Perhaps even every costume shown in every Toriyama artwork while they're at it, and perhaps even costumes shown in anime fillers while they're at it. (come on, free DLC is meant to add what you didn't have time to add before the release date)
Make this game the ultimate
Dragon Ball collection (not in number of characters, but in number of stages/appearances associated with the characters that end up being in the game). Having to unlock those by playing - again similarly to
Injustice RPG-like levels for example - would enhance the amount of time we're willing to spend to unlock our favorite versions.
But unlocking things is not everything, it still needs a story, certainly not "random senseless fights one after another to unlock things".
Good gameplay, good story mode, and good amount of interesting things to unlock (possibly: stages / story, artwork and filler costumes / show's BGM / manga colors / HD-remade openings and endings from the TV shows and previous games).
We don't need more that something nice to play with your friends, but also alone with an interesting story to go through and interesting unlocked things to play for).
Good gameplay, good story, good unlocking, and we're good to go.
