One of the things that seemed to plagued Enterprise was the fact that so much of the Federation was already known. How about some of the other powers involved?
Think of an animated series based on the rise of the Klingon Empire. Maybe a mini series based on some of the legendary Klingon characters of the past. Thier first attempts at space travel. First contact with a alien species. It could be interesting. And with a limited knowledge, but enough not to be spoonfed the basics, a prequel series could be done on them.
Maybe some other Trek species I'm not sure of? Perhaps the Rise of the Borg or the Dominion.