9) Captain Universe
Captain Universe isn’t so much a single character than a persona that can switch from host to host. The personification of the Uni-Power, an extra-dimensional force that imbues a person with just about every superpower you can think of, Captain Universe would make for a fascinating new addition to explore in the Guardians‘ cosmic universe.
Over the years, he has inhabited a number of hosts, including a bunch of pre-existing characters. Everyone from Doctor Strange to Spider-Man to Daredevil to Deadpool have wielded the Uni-Power on occasion. Captain Universe wouldn’t serve as a new member of the team, then, but probably function as the Macguffin of the movie, in much the same way that the Orb did in the first film.
Now that we mention it, the MCU will need a big overarching threat for Phase 4, seeing as the long-gestating Infinity Stones storyline will have been wrapped up. The quest to stop the Uni-Power falling into the wrong hands could easily fit the bill, and the arc could all kick off in Guardians 3 or 4.