Battlefield is earth.
Every character in DC and Marvel that you can imagine as a playable character or as a boss in a fighting game is valid or as an army card in the VS system as armies (ALL of the lanterns for example).
No preparation (prep in the mythos kills the fun).
The horror effect doesn't occur when you see them. The Mythos beings need to indoctrinate the characters, meaning to focus on a mental attack which may take times for the lesser ones.
Who wins and who are the major players in the combat.