Any number of the above suggestions for Carol Danvers (except Katee Sackhoff; I'm over it) I liked the pictures/ideas on page 2 of Anna Torv or Katheryn Winnick, or Yvonne Strahovski. Have her be the strong military type but skew somewhat younger. Think Carol Ferris in the Green Lantern movie, I suppose.
Also: a tiny bit older for Mar-Vell. I'm thinking someone what could be a badass as Mar-Vell, but also have that pretty scientist vibe during calmer scenes (or when he's Phil Lawson). Someone through out Aaron Eckhart as an idea, which would be cool. Or, if you want to go a bit younger, Stephen Amell or Jared Padalecki.
I like the picture/idea of Sam Witwer as somebody, and his amazing voicework in The Clone Wars (as The Son and Darth Maul) lend him to a villain. I'm thinking Yon-Rogg, or some other Kree that would be the main villain of the Captain Marvel movie.
The plot of the Captain Marvel movie should start with Mar-Vell as the titular role. The plot would revolve around his efforts to prevent the Kree from invading Earth (think Ultimate Secret, or the episode "451" of Avengers: EMH). During the movie, Carol would get her powers, and in the epic fight at the end, she would be Ms. Marvel alongside his Captain Marvel. She would, essentially, be the sidekick. Her costume might be based on the black swimsuit costume she's most known for (perhaps thrown on at the last minute, with at least one good self-referential joke about how silly the costume is), or could be similar to the one Moonstone wore in Dark Avengers. In the big fight, Mar-Vell sacrifices himself to save the Earth. In a post-credit scene, we see at least one member of the Avengers asking Carol for her help in dealing with Thanos (note that the Captain Marvel movie is between Infinity War 1 and 2). She is now Captain Marvel, and wears a costume resembling her current one. This general outline has the benefit of acknowledging her beginnings as Ms. Marvel, but ends where she is now, as Captain Marvel.