Black Panther: He's got cool powers, a cool costume, and an important place in the history of black superheroes. Plus, he's from Africa, and this team could use a little international flavor (and no, Asgard doesn't count).
Vision: Who doesn't want an android hanging around headquarters, particularly when that android can self-charge with solar power and then use that solar power to shoot powerful infrared beams at your enemies? He's badass and carbon neutral.
Falcon: Sure, he's another diversity choice, but Falcon is also one of the coolest flying Marvel heroes around. Plus, he's a longtime pal of Captain America in comic book continuity, so maybe it's time to bring that friendship to the big screen.
Scarlet Witch: She's got a very intriguing set of powers, she adds more girl power to the team and she's been an integral part of some of the biggest Avengers comic stories ever. Talk about fuel for sequels.
Luke Cage: Who doesn't love a guy with indestructible skin? He's been a big part of Marvel's New Avengers comic series lately, and he's got a New York attitude that would fit right in to future Avengers adventures in the Big Apple.