Fans like to imagine what it would be like if two pop culture universes intersected, bringing their favorite franchises together in an awesome movie or TV show that makes their geeky dreams come true.
Licensing restrictions and greed tend to keep these crossovers from actually happening, but there are quite a few geeky franchises out there that already exist in the same universe, even though many are unaware of the connection.
The TMNT and Daredevil seem like they're from totally different worlds, but they share an origin story- because the canister that left Daredevil blind and gave him powers is the same one that transformed four little turtles into the Teenage Mutant Ninja Turtles.
Transformers and G.I. Joe are about to cross over in the movies, but they've existed in the same universe from the very beginning- they're both owned by Hasbro, they had cross over story arcs in the comics, and both animated series' were produced by Toei Animation.
Spielberg and Lucas worked together on the Indiana Jones series so they naturally had a few crossovers, namely a kid in a Yoda costume appearing in E.T. and those E.T.-style aliens seen at the senate meeting in Star Wars: Episode I- The Phantom Menace.