How about another legendary idea: What if the Avengers and X-Men fought some supervillains instead of each other? "[Superhero team] vs [superhero team]" seems to be one of the main recurring plots at Marvel these days. And it does largely seem to be a Marvel thing; I mean, when was the last time an event revolved around the Justice League fighting the Justice Society? They did already and it was less than impressive, partially because they let the readers vote on the outcomes of some of the battles. The Amalgam Universe it produced was fun, though.