so, what would make for an intriguing 'mega crossover' but the antagonists would have to change universes-- for "Crisis on Infinite Earths", the AntiMonitor would menace the Marvel multiverse-- for "Civil War", one faction of DC heroes would advocate for national registration, and another faction would resist. For "Invasion!" the Dominators would attempt to invade and take over Marvel Earth. For "Secret Wars 2015", maybe Darkseid somehow is granted ultimate power over the multiverses, and the DC Heroes are stuck in new interpretations of themselves until they 'wake up'...
Could be fun, could be crazy, could have some intense conflicts..