1) The comic books and other media we get are actually fictionalized/disguised versions of events in a real universe. They probably have as much relationship to real events and people as the movie Tombstone does to the OK Corral and the lives of the Earp brothers.Here are some examples of how that would be put into action:
2) There is one, primary Earth. All the Marvel and DC heroes (as well as a number of other comic and pulp characters) inhabit this world.
3) This world is as "real" as our world, except for the inclusion of superpowers and what not, so people and institutions have behaviors and motivations much more similar to what we see outside our window than in the pages of kid's funny books. Also, "realistic" means people age; no sliding timescales.
Does there just happen to be two brash bowman who wind up with blonde girlfriends with sonic screams? Nope. Green Arrow is a legacy hero, and "Hawkeye" is the original's sidekick grown up, who was briefly a villain, then an authority-questioning hero. he didn't pay enough attention to his side kick, and the kid fell into drug abuse, but eventually gets clean and becomes a SHIELD agent.
Or, here's the true history of some reptile-themed villains: A scientist named Curt Connors, desperate to help injured vets like himself, sets up a special clinic in the Florida giving an experimental treatment--with tragic results. One of these before doomed veterans gets his wife pregnant, and the child grows up to show latent genetic damage and enters a life of crime:
Anyway, you get the idea. It's amazing how many of connections like these you could make, and I think it would make for a fun campaign with a lot of room for creative (at least in "discovering" connections) with the advantages of using established comics universes.