I must admit I liked the sound of the 'Star Trek: Federation' idea that was nearly but not pitched to the studio. I like the idea of dealing with a federation that is in decline.
I'd also like to see a series set in the far future, with the 'temporal directive' or whatever it's called in force. Perhaps explore in depth the characters and concepts from the far future that we meet in Enterprise in real depth.
Personally, I like the darker story lines so I'm all for a bit of doom & gloom. I'd like to see more moral ambiguity in Trek.
Perhaps a spin-off series set in 'Section 31'. I guess this could be any era really.