2. Illuminati
For those who aren't familiar with the Illuminati (not the ones from those Tom Hanks movies, the Marvel ones), it's a secret super group comprised of the most influential super-beings on the planet. While not a full time group like the X-Men or Avengers, they meet periodically to discuss and plan anytime there is a potential threat to the planet. At various points in time, the Illuminati have included the following: Iron Man, Captain America, Black Bolt, Black Panther, Namor the Sub-mariner, Beast, Doctor Strange, Mr. Fantastic, Medusa, and Charles Xavier. For the sake of the MCU, the most likely lineup would be Iron Man, Cap, Black Bolt, Black Panther, Doc Strange and maybe bring in Namor or Reed Richards (although it seems almost silly if there isn't going to be a Fantastic Four reboot). We have to assume the final Avengers movie in phase three will be the ultimate culmination of everything that has happened up to that point. What better way to segue into this that with a movie about the Illuminati reluctantly banding together to develop a plan of action against a threat so big, it warrants their collective involvement. The Doctor Strange, Inhumans, and Black Panther movies could beautifully set-up the Illuminati flick. This would, in turn, prepare us for the big finale. In the comics, the Illuminati first form in response to the Kree-Skrull war. In the MCU, they could form in response to the battle with Ultron, or in anticipation of whatever threat shows up in Avengers 3. Either way, what true geek wouldn't get goosebumps the first time this group all sat together at a round table and started discussing things?