Characters in Marvel Comics have rubbed elbows with just about everybody, from superheroes owned by other publishers, such as DC Comics, to celebrities and presidents. Celebrity cameos in Marvel Comics are more prevalent than you might think, including late-night host David Letterman and rapper Eminem. These aren't alternate-universe stories, either: The majority of Marvel's strangest crossovers have happened in canon.
As the MCU starts to move past some of the more obvious storylines, almost any crossover is possible. Still, there's basically no chance Marvel's most insane crossovers will ever show up in a film - and not just because of legal issues like rights to characters. Some of these crossovers that have happened in the comics are just way too weird even for the MCU.