The film industry is currently littered with cinematic universes, from Marvel to DC to Star Wars to even the likes of LEGO, and there are many more in development, like Call Of Duty, Hasbro, and Valiant Comics.
It really does feel like every major studio is desperately scrambling to pull together their own major movie crossover, and while there are many successful examples of this, there are even more that have fallen by the wayside over the years.
Mashing two separate franchises into one movie is a logistical nightmare, so it's not surprising to learn that many of these attempts fall apart early in the process, and some of the time, barely get further than the "initial conversations" phase. Rights issues, differing tones and actor salaries are just a few of the hurdles facing these budding crossovers, problems that - generally - prove impossible to overcome.
So while Batman fighting Iron Man would be awesome, and Sherlock Holmes schooling Harry Potter would win all the Oscars, it's really not as simple as signing a few contracts, and these seven failed movie crossovers can attest to that.