Garrett Hedlund as Richard Rider/Nova
Not a particularly creative fancast but one I hope gets done, Nova is pretty much set to appear anytime between GOTG and GOTG 2. Hedlund is an underrated actor that I feel should get cast in a CBM sometime soon.
Honorable Mentions:
Jared Padalecki (very close second)
Nathan Fillion
Michael Jai White as Luke Cage
I usually see The Rock or Terry Crews for the role of Luke Cage but I'm surprised I haven't seen anyone go with MJW. Originally my pick for Black Panther, I see alot more Luke Cage in White. He's no stranger to action as he's played Mike Tyson, Jax from Mortal Kombat and he's an accomplished martial artist. While he would be good for either role I can definitely see him being the Hero for Hire.
Dave Franco as Alex Powers/Powerpax
A very promising leading man who can without a doubt contribute to the comedy of a movie, Franco is my (slightly older) choice for Powerpax. I choose Franco because I doubt Marvel will work with children unless it's from the say-so of a star like RDJ or a necessity. My bet is they cast a more mature Powerpax and I think Dave could do the job. (He's already played a high-schooler right?)
Jayma Mays as Rita DeMara/Yellowjacket
Jayma is a solid actress and considering the Ant-Man film is taking place after Hank Pym's run as Ant-Man then they should have Rita DeMara, who stole his identity at one point and is known as the villain Yellow Jacket.
Ryan Gosling as Captain Marvel
Gosling is a glossed-over actor that people don't usually consider for a CBM because he tends to do romance films, big budget movies and comedies. However ,with the rise of the CBM and the second, third and fourth highest grossing films of 2014 all being comic book films and with no signs of slowing down, I think it's only a matter of time before Gosling enters the fray and portrays a character. If so, I think he would make an excellent Captain Marvel. This was not a easy decision by any means but Gosling is an actor that I think commits to his work and would do his best to give us a faithful adaptation of the character.
Honorable Mentions:
Ben Foster
Alexander Ludwig
Chadwick Boseman as Black Panther
I admittedly had not been impressed by Boseman as an actor and had not considered him for the role until Draft Day and then being impressed with his presence after seeing the trailer for Get on Up in theaters. This convinced me that he should be given a shot as he's got alot of publicity under him right now and could bring star power to the film.
Willow Shields as Cassie Lang/Stature
This would be a necessity. Cassie Lang is a great character and will no doubt appear in the upcoming Ant-Man film. If the character is to stay and eventually become Stature then you can't really do better than Willow Shields who is an accomplished actor and is no stranger to the business.
Olga Kurylenko as Agent Anesthesia
Olga is a good actress that can play Agent Anesthesia, the secondary villain in the film who will serve as the personal quarrel for one of our heroes considering she works for AIM to aid her father who is in a coma.
Emma Watson as Jeanne Foucault/Finesse
A polymath with the ability to instantly master any action by looking at it once. It is suspected that she might be the daughter of the Taskmaster. Emma Watson is a proven actress that, in my opinion, is looking for that next breakout role in her career. Playing and popularizing the character of Finesse will offer her up future roles and expand her filmography while also adding even more star power to Marvel.
Dustin Clare as Taskmaster
With work on shows like Spartacus he certainly has the physicality to portray this villain and his acting chops have been proven to be more than okay. Should Marvel choose not to go full-on CGI then Dustin Clare would be my pick for the man villain of a Marvel hero solo movie.