The LEGO Movie is a real thing that is happening. Mostly generic Lego people lead by Morgan Freeman as Clearly Not LOTR Lego Gandalf, but also DC Heroes and Ninja Turtles - as well as the Renaissance painters they’re named after. See also Doctor Who now having Star Wars scale toys, so this can happen.
How would you approach something like this in gaming, a crossover between disconnected settings? Your urban fantasy monster hunters teaming up with another table’s dungeoneers and another day’s superheroes?
Obviously you need to get a few compatible groups together, or at least bits of them. If a player is in a couple of the games involved, they choose who to play.
The highest native Weird Level probably becomes the minimum, which may stretch credibility for lower-weird settings. James Bond would have a hard time accepting Thor as real, but not vice versa. The Transformers meeting the GI Joe team would be a big deal for the latter, not the former, so it happened outside regular continuity for either. The Seventh Doctor fought Death’s Head after the Transformers did and before he entered the mainstream of the Marvel Universe, but neither of them likes to talk about it. So consider what, if anything, is to be counted as canon in future sessions.
And a big enough threat is required. A reality storm that could destroy multiple universes, say?
And you need room for at least one “the heroes fight over a misunderstanding” scene. Because if, for example, you wanted to set up an urban fantasy crossover Avengers, Buffy and Hellboy just have to fight before allying to fight Voldemort.