I too would love to get a book set after The Soul Key that actually resolves the Ascendants. We've gotten a bunch of hints, but I want the whole story!
My vote, though, would actually go to stories set within the TV shows TNG, DS9, Voyager, and ENT. I think if you were clever you could find some really interesting spaces to fill...in particular, I'd LOVE a book set in between the first two or three episodes of TNG, retroactively connecting character arcs to the more stilted writing and performing back then. It seems weird to me that TOS books set within the 5 year mission do so well, but they're not publishing any similarly-set 24th century novels.
Yes! If only because I happen to have a Voyager story set around the first few episodes I really want to tell at some point.
We're hitting TNG's 30th in two years, then it's rolling for the others from then on. Maybe a good point to do it, even if they're one-offs for the celebration.
I went for New Frontier, because it's the only one I've read even though that's just six books. Plus from what I've gathered on here, it doesn't end properly compared to something like Vanguard.