And no, I'm not talking about the Amalgam universe here.
DC and Marvel have had quite a few crossovers: Superman vs The Amazing Spider-Man, DC vs Marvel (or Marvel vs DC, depending on the issue), and JLA and Avengers. However, I'm proposing an interesting theory: what if there's a universe in which DC and Marvel characters exist together?
Now, this theory is based around three comics: Superman vs The Amazing Spider-Man, Spider-Man and Batman, and Batman and Spider-Man. You see, in these comics, the characters crossover just by going to a different city, implying they live in the same universe. Also, in Spider-Man and Batman, Spider-Man fanboys a bit after meeting Batman, and Carnage expresses admiration for the Joker. So, obviously, they know of each other. Now, the two Spider-Man/Batman crossovers are under the Elseworlds imprint (basically DCs way of having miniseries that take place in alternate timeline), so it's not the normal continuities for these characters.
Now, you may be wondering, where does Superman vs The Amazing Spider-Man fit in? Well, much like the Spider-Man/Batman crossovers, the two heroes meet by going to different cities. Now, I think that these three stories take place in the same timeline. I could completely prove it, except for one thing. During Spider-Man and Batman, Spider-Man says that he's teamed up with a lot of well known superheroes, and he wishes that "Superman would call." Now, he could be joking, but I don't think he is. Now, this might prove my theory to be false, except for one little thing: Crisis on Infinite Earths.
Now, I don't know much about Crisis, except that it's a reboot of sorts of the DC universe in the 1980s. So, perhaps there was a Crisis in this shared DC/Marvel timeline, rebooting it in such a way that Spider-Man has not yet met Superman. Ergo, Superman vs The Amazing Spider-Man can exist (sort of) in this timeline.
Naturally, this is just a theory, but, you have to admit, I do have a point here. If anyone sees any flaws in my theory, please let me know. Until then, let's find a designation for this universe.