Mr Darcy vs Heathcliff

I've been musing on the appeal of the 'team-up', a type of comic-book story where characters from different series meet up for a fight and/or adventure. Often there is an unlikeliness to the combination - for instance, characters from different genres (superhero+supernatural, eg Spiderman and Brother Voodoo; superhero+western, eg Batman and Jonah Hex; superhero+war, eg Batman and Sgt Rock), orientations (good+evil, eg Batman and the Joker), fictional+real (Superman vs Mohammed Ali, Jimmy Olsen and Don Rickles); or different times (eg Spiderman and Red Sonja who is of course from the Hyborian Age). Sometimes the difference is more one of style - a serious character with a light-hearted character (Batman and Plastic Man), a major character with a minor one and so on. Batman and the Shadow was interesting - a character meeting his literary ancestor (a bit like Tarzan meeting Mowgli.)

A special category of team-up is the crossover involving characters from different companies. The first of these was 'Superman versus Spiderman', (characters owned by DC and Marvel respectively) in advance of which I could barely sleep for anticipation. I mean, they were from different universes which worked to totally different rules and had different histories - how would this be explained? How can you have an America with Metropolis, Gotham (DC) and all the regular cities (Marvel)? In the rather tame story this was simply ignored though later Marvel/DC crossovers invented some parallel-universe schtick but it never seemed particularly satisfactory.
I'm surprised the team-up concept hasn't spilled over into other genres - soap characters showing up in different soaps for instance. Or of course real literature - a sort of hard-wired intertextuality where instead of referring or alluding to other texts the 'actual' characters 'really' meet up ('Not a dream! Not an imaginary story!'), bringing their associated themes, paradigms, worldviews, metaphysics, authorial voices etc with them to clash, mingle and collapse in the background.
I have to deliver a session with some recently-merged academic departments next week - perhaps my years of reading team-up books will give me a rich seam of lore to draw upon... comments like 'remember when Superman lost his powers and teamed up with Swamp Thing' will no doubt give us some common ground for discussion.