Now, here's the thing about Marvel and DC. For all that we all love to wage war over which company produces the superior comic books (and now movies), the two corporate entities (owned by Disney and Warner Bros. respectively) aren't actually in the habit of displaying all that much animosity towards one another. Which, seeing as the two companies have shared a whole lot of staff over the years, isn't really all that surprising: At the end of the day, they're business rivals, not mortal enemies.
So much so, in fact, that on a number of occasions over the past 40 years, the two companies have actually managed to pull off some surprisingly successful (and, crucially, extremely lucrative) crossover events. The big question that raises, in the age of the Marvel Cinematic and DC Extended Universes?
What Would a Marvel/DC Movie Crossover Actually Be Like?
Well, for one thing, there's a pretty good chance it would be an awful lot like the recent crossover between The Flash and Supergirl - fun, but determinedly finite:
Specifically, it would almost certainly be a pointedly "one time only" affair — one that would likely be result of several years of dwindling box office takings for both the MCU and DCEU. That, after all, is the context in which the majority of previous (comic book-based) crossover events have taken place. They're not the sort of enterprise you embark upon when you're feeling entirely confident in your own properties, after all.
More specifically, though...
There's A Pretty Good Chance That There's Only One Sort Of Crossover That Could Ever Actually Happen
Specifically, that one, right there: Amalgam. Y'see, as part of 1996's DC vs. Marvel crossover event, the two companies decided to mash together their most iconic heroes into strange and often glorious combinations, as part of the co-owned Amalgam comics. Now, the (entertainingly ridiculous) main plot line of DC vs. Marvel ultimately both created and destroyed that universe — one too perfectly silly to survive for very long. But while it lasted, it offered us an intriguing glimpse of a way in which Marvel and DC could cross over their respective movie universes without compromising their respective comic book movie properties.
Y'see, one of the biggest stumbling blocks standing in the way of a Marvel and DC movie crossover is the fact that such a project could well damage (or be perceived to damage) the self-contained nature of the MCU and DCEU, as well as somehow stunting audience members' ability to suspend their disbelief.
Dive headfirst into an Amalgam-style mash-up movie, though — one in which beloved DC and Marvel heroes don't technically appear, instead having been replaced by combined versions of the original heroes and villains — and those problems might just be avoided. The inherent silliness of the enterprise would, hopefully, be limited to the movie itself, rather than spilling over into the MCU and DCEU. There'd be no particular reason to see the original universes as having had their self-contained natures compromised in any way.
Plus, if Marvel and DC managed to get Fox on board too, we could finally see the Batman/Wolverine hybrid (above) we've all spent the past two decades dreaming of seeing on the big screen — and there are plenty more where that came from.
Now, of course, odds are this is all just a pipe dream and that it'll never even come close to happening, but that doesn't mean it's not a pleasant day dream to ponder.