Would have to have darkseid as main antagonist of the game, probably basic invasion plot with stops all around the DC universe, have heroes fighting on earth and other planets in the DC universe before going to confront Darseid on Apokolips only to discover he has taken over new genesis and is building some kind of death star type weapon there.
Playable Characters:
Batman
Superman
Wonder Woman
Flash (alternate skins for Jay garrick, wally West)
Green Lantern- Hal (alternate skins for Alan,Kyle, Jon, and Guy)
Aquaman
Mera
Cyborg
Martian Manhunter
Hawkgirl (Kendra as alternate skin)
Zatanna
Green Arrow
Black Canary
Shazam
Batgirl- babs (cassie and steph as alternate skins)
Robin- Grayson (Nightwing, Red Hood and Red Robin as alternate skins)
Starfire
Supergirl
Steel
Katana
Black Lightning
Mister Miracle
Big Barda
Orion
Bases of Operation
Hall of Justice
Batcave
Fortress of solitude
Thymescirya
Flash Museum
Atlantis
Oa
Levels
Gotham City- Clayface as boss fight
Metropolis- Braniac as boss fight
Atlantis- Ocean Master as boss fight
Thanagar- Steppenwolf as boss fight
Korugar- Sinestro as boss fight
Khandaq- Black adam as boss fight
Apokolips- Kalibak, Furies, and granny goodness as boss fight
New Genesis- Darkseid as boss fight
End Credits scene sets up Trigon as villain of possible 2nd game