I think with this movie completing an epic trilogy and the Avengers bringing several movie plot-lines together, that Hollywood should focus on something else. Leave the superheros alone for a while. Over the past decade we've had more than enough (despite the box office declaring otherwise). And introducing more and more characters or more outrageous situations is going to take away from the novelty of each individual origin story and make each character static (if they aren't already...I'm looking at you Iron Man).
I want Hollywood to focus instead on Manga and Anime. There are plenty of interesting settings, characters and story lines that could be...well let's be honest...exploited. My favorite of which is an epic Space Opera in the same vein as Star Wars, but much, much more mature (as in intellectually stimulating) and without all the different species. It's called "Legend of the Galactic Heroes" and despite it's stupid name, it has all the meticulous fantasy of a Tolkien book, the political and moral depth of Plato's Republic, and the action of the worst Michael Bay films. The story has it all: revenge, political domination, coup d'etats, military occupation, diplomatic impunity, religious zealousness, insurgencies, and assassinations - all on different planets or in space.
But I understand if you are not excited by this prospect as much as I am. Anime can be so bland at times, with it's formulaic approach and over the top actions or reactions. Even Miyazaki, the Animation God, is guilty (and maybe the reason why others still follow those guidelines). Hollywood could streamline it though and focus on the story, characters, and settings. They could do what they did for the comic book industry to the manga industry.
As much as I would want to see Apocalypse and Magneto face off in another X-Men, these super hero movies have nowhere to go except for mindless fun.