Hank Pym has always been a central character to the Avengers. With Edgar Wright still attached and Kevin Feige heavily hinting towards this the appearance of Ant-Man and Wasp, this one appears like the most likely one to appear, but is he your favorite?
T'Challa has been hinted at in a couple of the films already. Kevin Feige has shown interest in the character, and would add a lot to the Marvel Cinematic Universe. Hopefully it'll get a good actor and director attached to it. I for one am looking forward to this one in particular.
Thor was a tough character to translate into film for a mainstream audience, but Kenneth Branagh somehow pulled it off. Now the next challenge for Marvel is to make Dr. Strange a success. Out of all the Avengers, I think this is perhaps the hardest character to sell. But Thor is proof that it can be done. Is there at least enough fans out there wanting to see this movie to convince Marvel to move ahead with a Dr. Strange solo film? Perhaps a Mandarin appearance in Iron Man 3 could serve as a segue into a Dr. Strange movie.
The lovely Ms. Marvel has been rumored to have a show coming up. Is that enough? I feel that the Avengers could certainly use more female members, and Ms. Marvel is my favorite. I sure hope her solo film isn't too far off.
The Sub-Mariner would add some interesting dynamics to the Avengers as well as introduce a very different atmosphere for a film in general. Can a aquatic superhero ever be done right? Let's hope so because Namor is just plain awesome.
Pym's wife would be a nice addition to the Avengers and would naturally appear along with Ant-Man if Ant-Man is made. The question is, is she more of a favorite to fans than her husband?
Finally there's Vision. He's probably the less talked about hero that has been a common member of the Avengers. The android's introduction would also probably include the involvement of Ultron. Is he popular enough to warrant an appearance? Also, could he even hold his own in a solo film? Perhaps further down the line.