Another crossover I think would have been fun would have been Aliens/Halo.
Finding ovomorphs on a Halo ring having been studied by the Forerunners (as per the flood); imagine how full-Kenner you could go with that one, with the xeno variants burst from hosts of various Covenant species (or even Flood-xenos) - UNSC tech isn't that far removed aesthetically from USCM; could be a visually beautiful comic book.
I think that's my biggest gripe with the superhero crossovers like Superman vs Aliens and AVA etc... it's not the idea of crossovers that are the issue, I think it's more finding the *appropriate* crossovers.
