I'd love the idea of an Ent-C series, but it would be bitter sweet as you know how it ends. Going back to the B would give you a pretty much blank canvas, after all there are only two established characters aboard. I love both the Excelsior- and Ambassador-Classes, so any chance to see more of these two ships is a big plus for me.
However, I would like to see a post Dominion War series, maybe along the same lines of New Frontier - a ship deep inside Cardassian space on a humanitarian mission. We saw the war and how bad things got, but we never got to see the hard work of rebuilding all that had been lost, the anger and bitterness the war had caused, the loss and grief of the survivors. I think it would make for an interesting and compelling series, the crew have a job to do whilst dealing with a lot of their own issues and pain, over-worked and under-staffed.
-Bry