4. Black Panther
In their current state, the Avengers aren't exactly the most culturally diverse group of superheroes out there. Even Hulk is a white guy most of the time. King T'Challa of Wakanda brings a boatload more than just a minority perspective to the MCU. A Black Panther solo film offers writers their choice of great stories to showcase in film: T'Challa's rise to the rank of Black Panther, battles with Man Ape and the White Gorilla cult, the Wakandan civil war etc. As far as fitting into the rest of the cinematic universe is concerned, there may be no better character than Black Panther. As a head of state, he would have had some official say in the events that take place during Avengers and/or other major events. In the comics, Black Panther becomes a member of the Avengers and is also affiliated with the illustrious Illuminati. Black Panther is one of the few characters who can make even the likes of Tony Stark and Captain America listen when he speaks and rules the only nation in the world that produces vibranium (the metal from which Captain America's shield is made). T'Challa is a strong king, a respected diplomat and a badass warrior. 'Nuff said.