Guardians of the Galaxy is a movie I'm looking forward to. When the news first came out at the 2012 San Diego Comic Con that GOTG would have a spot in Phase 2 I was a bit skeptical, but now I trust that Marvel won't let their fans down.
A few months ago Joss Whedon, the director of Marvels The Avengers, revealed that Thanos was being saved for the big finale, Avengers 3. If Guardians of the Galaxy is being released in Phase 2 and Thanos won't be the villain till the end of Phase 3, then why not have GOTG in Phase 3? My prediction is that Marvel plans to have a sequel of Guardians of the Galaxy set in Phase 3 and build a franchise on this lesser known comic-book team.
Guardians of the Galaxy could birth more than just sequels. By introducing different characters and species in GOTG movies, Marvel could start spinoff franchises like Nova, The Inhumans, and Captain Marvel!
This is my theory for the future of the Marvel Cinematic Universe. Comment below which cosmic hero you'd like to see in an MCU film.