If there's one Guardian who should go from a GotG movie to an Avengers sequel it's him. I was surprised he wasn't announced at comic-con.
There are a couple of reasons I can think of why he wasn't
1) Maybe they're saving him for a sequel? (the most mundane)
2) Maybe he'll be featured in the movie, but never join the team. This would allow him to travel to Earth at the end to warn the Avengers about the coming of Thanos, while leaving the team, intact, operating in space.
3) Maybe Magus will be the villain and, in a bit of a departure from the comics, they have to "purify" him, to save Adam and the universe.
4) Maybe he'll be trapped in the soul gem.
Anyway, I hope he shows up, and I've got two casting choices for him