Insurrection nicely set up a series that could have revolved around an internal rift within the Federation. Rather than looking for a new enemy for them to face, they could have had a civil war or sorts.
Gene Roddenberry already came up with the idea of a Federation civil war and its aftermath. That series was called Andromeda...
Well, sort of in very general terms.
You could take Andromeda
and re-shape it as a Star Trek
sequel, but it would take a bit of tweaking to do so. But, I do not believe Roddenberry ever intended the concepts/ideas that became Andromeda
to be any part of Trek