I don't think so. It would work better if The Avengers films served as sequels to all of them. Just works better. Especially for Thor and Iron Man. Their rogues gallaries don't lend for more than one solo film easily. Way I see it, have a solo film for Cap, Thor, Iron man, and Nick Fury. Then, do an Avengers movie. I'd base it quite heavily on the Ultimates, simply because it's an origin story that would lend itself well to apeasing the mass audiences, and brings the characters who haven't had solo films (Giant Man, Wasp, Hawkeye, and Black Widow) into the mix nicely. 'Course, I'd make a few changes to fir with the solo films a bit. And, I'd probably leave out Quicksilver and the Scarlet Witch. Since they weren't introduced in the X-Men films, fitting them in would be rather dificult.