I'm just posting all of the ideas from a book that details the various planets in the UFP. There's some pretty interesting ones in there, and it could give us some material to work with if everyone gets stumped and can't come up with their own.
Of course, we could always do a holodeck adventure or two, but that would definitely need some discussion to figure out what genre we'd want to go with, what the adventure would entail, the personas in the storyline our characters would assume the identity of for the story, etc.
As for the Temporal Backlash idea, Mr. Dillard actually deserves a LOT of credit for combining the ideas (as presented in post #4 on this thread).
Something I proposed to Mr. Dillard would definitely result in someone being unhappy with the events surrounding and following the accident that destroyed the Starbase and the Zealous. It would involve a character "dying" in the explosion, and a former Zealous crew member realizing, sometime in the future, that they had feelings for that character who perished.
Add in there the prolonged war with the new race, which would have devastated the Federation and other species, and it would be the perfect catalyst. It would facilitate the selected character getting the remaining Zealous crew together, and defying direct orders from Starfleet in order to go back in time and prevent the accident from taking place.
The new races that I have in mind, the write up for one of which is 90% complete, would both be enemies of the Federation. We would start with the one, and eventually work in the second one over the course of several storylines. Depending on what everyone else comes up with in the way of new species, we could even add in a few more.
Just like in TNG, we could do a couple of storylines which would deal with these races, and then take a break for a storyline that might be fun . . . or something totally unrelated. After that, we could always go back to dealing with those races.