5. Agent Phil Coulson
Seriously, we just need to know he's not really dead, right? Right??!! Clark Gregg has rightfully earned himself a cult following for his portrayal of the very likeable and very funny Agent Coulson. If Coulson doesn't appear in the actual main body of the movie, even just a wry smirk from him in the after-credits scene would put his fans' minds at ease. The sooner that happens, the better. His apparent death in The Avengers served as the catalyst for the heroes coming together as a functional, cohesive team. It would be typical of Joss Whedon, and satisfying for viewers, to see him return, having found out that the Coulson who 'died' was actually an LMD (a Life Model Decoy - the existence of which was actually hinted at by Tony Stark in his conversation with Coulson earlier in the same movie) and that the event was manufactured by Nick Fury in order to get the Avengers to work as a unit. Joss likes killing off people, only for them to be brought back to life, as proven by numerous revivals in the Buffyverse. It would be no surprise to find out that Coulson didn't really die in The Avengers. With the SHIELD spin-off TV series definitely happening, Coulson has the potential to be the main star of it, and he surely has a bigger part to play in the MCU, so he simply has to return.