First up is T'Challa, Otherwise known as Black Panther- ruler of Wakanda. He needs to be a big part of [The Avengers: Age Of Ultron](movie:293035) if they are filming in South Africa (which may double as Wakanda.) The main source of Vibranium should be a main plot line if they are fighting Ultron. He was a member of the Avengers in the comics when they fought Ultron and he should be now as well in the Cinematic Universe.
Ms. Marvel, otherwise known as Carol Danvers needs to be introduced before the 3rd Avengers movie. She could have a cameo in some other phase 3 movie, perhaps Guardians of the Galaxy(2) which would introduce the Kree backstory. The Avengers will need her powers to battle Thanos. (They will need all the help they can get!)
The 3rd is Wonder Man, Otherwise known as Simon Williams. His body was nearly destroyed by the Ionic Energy that gave him power, leading Hank Pym to build an android to house his mind (which later became The Vision.) Hopefully, The Ant-Man movie will include some backstory on Simon Williams (in the 60's) They could have him in cryo-stasis until Pym figures out how to stabilize his powers (in the present day) and bring him back. He also needs to be a member of the Avengers when they battle Thanos. This assumes that Vision will be introduced in Age of Ultron as well as the new members we already know of, Scarlet Witch, Quicksilver & Ant-Man (Scott Lang.) The team will be a little crowded by [The Avengers 3](movie:738027) flick, but that's OK with me.