DC Entertainment is doing its darnedest to translate its similar hodgepodge of supercharacters onto the big screen, planning to add familiar heroes and heroines to its next Superman movie, mainly Batman and Wonder Woman, no doubt laying the groundwork for an eventual Justice League movie. We've already seen a bunch of characters from DC Comics (Flash, Aquaman, and more) pop up in the television series Smallville and Arrow.
Superhero movies aren't the first to invent crossover mashups. Universal did it way-back-when during the peak of its hit monster movies. Audiences loved seeing Dracula, the Wolfman, and Frankenstein's Monster join forces (or clash). Some viewed it as the end of a cycle -- when individual franchises run their course, the moviemakers try to milk some more money out of them by combining them with other titles. It's still a practice that happens today with Freddy vs. Jason or Aliens vs. Predator. Marvel has shown, so far at least, that its shared universe still has plenty of momentum left with no sign of slowing down, at least in the forseeable future.
There are other shared universes out there full of possibilities. Godzilla in the past has collaborated with Mothra, Rodan, and other giant monsters, even with King Kong. Stephen King has a knack for including hints in his novels that imply that all his stories and characters are around in the same storytelling universe. Quentin Tarantino has done the same for his movies. Some even hypothesize that all the Pixar films co-exist, so there's a possibility for a mega-animated-teamup if someone clever enough can imagine a storyline that would work.
An obvious example of multiple characters existing in the same universe is the bounty of police procedural spin-offs. Maybe someday we'll see a feature film that united all the disparate men and women of C.S.I. or Law and Order.
Maybe everything is connected. Maybe all the characters in all the stories ever captured on the silver screen are dreams from the mind of Tommy Westphall, the autistic character from St. Elsewhere played by Chad Allen. Look up that theory for some mindblowing speculation about shared universes.