Since they can't include a Marvel world as Marvel is a multiverse (the best they could do would be to create an original world populated with the 21st century's most popular/well-known Marvel characters: i.e. X-Men, Avengers). Outside creating a whole world, they'd have to identify the most suitable candidates who wouldn't on their own break the game 'cause they gotta know fans aren't going to accept them being powercapped. Cyclops, Beast, Gambit, Emma Frost, Spiderman, Captain America, Hawkeye, Black Widow, Thing, Daredevil, Electra, etc.

Then they'd have to determine who among those character are popular enough to bother putting into the universe. Probably not DD or Electra having been received poorly due to horrible acting.

What Marvel villain could appear to justify the use of a "Marvel world"? There are so many choices. The obvious would be some universal threat like Galactus (which also makes sense as to why the group would bother fighting him as their priority has always been saving the worlds).