An open-ended online Marvel/DC team combat game. Some kind of mix between Freedom Force's combat style and the best of X-Men Legends- not quite an RPG, but with depth and devoid of the annoyances that plagued the combat of other Comic Book games. Allow team attacks, especially for characters from different universes.
Not that there wouldn't be a single-player and cooperative mode, but every player chooses one character in an online match and the teams are set loose in some large location (New York, Alaska, the Sahara, etc.) to find and destroy the other team. Apply Justice League Unlimited Physics for maximum property damage. Absolutely everything can be destroyed by the punches and kicks of the superstrong characters. Buildings, lamposts and cars can be tossed at enemies for good damage. Trees realistically burn, wither and collapse if you're Pyro, concrete freezes and cracks if you miss your shot as Iceman. Have at least 50 characters from both universes.
Obviously online play would require both careful play balancing for some characters (Superman vs. The Shocker? Carnage vs. Robin?) AND heavy smacktard-fumigation like all online games. Say we use Battlefield's voting system- after a round, a team can votekick idiots before the next one.