Guardians of the Galaxy..do i really need to say anything. This movie is going to be so good in my opinion hell it might even be better than Thor 2 but here's the question..What are they going to do for the sequel. Personally i hope they don't add too much important characters as the focus should be on the Team and having too many characters in the sequel would just ruin it. Here are the characters i think would work in the sequel and their role in the sequel.
Iron Man
No i don't think he will be in the first Guardians of the Galaxy Movie however i do think he might be in the sequel in order to protect the Earth from Thanos. Iron Man is defeated by Thanos forces and stuck in a Prison until the Guardians attack the ship and free Tony. Tony makes a deal with them. He will help the Guardians take down the Church of Truth as long as they help him fight Thanos. Star-Lord agrees and Tony joins the Guardians as Iron Man. Since we are most likely not getting Iron Man 4 in phase 3 i think it would be cool if Iron Man showed up in GOTG as long as he does not overshadow the rest of the charcters
Captain Marvel
Considering the push that Captain Marvel/Ms Marvel has been getting in Marvel NOW. I would not be surprised if she became one of the members of the Guardians of the Galaxy. I think they are likely going to set up her in GOTG2 which will lead into her own Solo Film later down the line. As for her back-story, well she is a Astronaut whose ship is attacked by the Free. Thought to be dead the Kree decide to use her by mixing her DNA with a Kree's DNA. Brainwashed to serve the Kree, she attacks the Guardians but they manage to hold her off. Star-Lord discovers she is being brainwashed and she manages to break free. Thankful for the help, she decides to join the team in order to fight the Kree.
Angela
For those who are wondering Marvel Comics had completely bought the rights to Angela from Gaiman so yes she can appear in a Marvel Movie. I think it would be the right time to introduce her and her race..The Angels as a species created by a creature known as the Beyonder. Angel is now a agent of the The Universal Church of Truth. A group of people who want to purge the world of sin and Angela is their top agent. She along with the Church of Truth fight the Guardians multiple times along with the Free. Later on Angela decides to help the Guardians fight Thanos but only because she knows just how dangerous he is. She is a Anti-Hero not working on the good side and betrays the evil side ( The Church of Truth) in order to work on her own.