Well, there's alot who should, but here's my top picks, really. ^^
Loki! I don't know about ya'll, But I kinda wanna see what happened during loki's exile. How he met the chituari and all that. That part of phase 1 is past though unfourtunately.
Hawkeye! I adore this character soooo much, and he needs a solo movie. I wanna see his origin story on the big screen too!! Its nit tonlate, Marvel!
Black widow!! She's a badass female hero, and we need more of those in cinema. Enough said.