Re: How about a solo Avengers franchise?
Marvel certainly will keep introducing characters through solo movies and expanding the universe, though I think doing so under an explicit "Solo Avengers" moniker would seem too much like they were trying to exploit it.
I could see that concept working as a TV miniseries though. Imagine a 10 episode run that takes place in the Marvel universe, with each episode introducing us to different heroes. It could be a lead-in for an Avengers film that featured all of them in the Civil War plotline.
Not that I'm arguing that the Civil War plot should be the focus of an Avengers film, just that if it were, the world of heroes would have to be greatly expanded, far beyond what I think Marvel Studios would be capable of doing just through movies.