1) More Federation politics! I eagerly await 'A Singular Destiny'. I find the set-up and workings of the Federation, such a large interstellar organisation, to be fascinating.
2) I would love to see more of life on Earth and the inner worlds of the UFP. We've actually seen relatively little of Earth in the 24th century, outside of San Francisco. 'Star Trek' always seemed to show a rather unusual living arrangement on Earth - lots of the places never seemed to have much technology in them. It'd be nice to explore this, though I recognise that it's unlikely because on the paraside that is Earth there probably isn't all that much story potential, beyond whenever there's a DS9-style challenge to that paradise.
3) 24th century Romulus is crying out for greater examination, I think. I also think it's crying out to be rescued from some of the blandness that was injected into it by 24th century Trek. I know Diane Duane has done a lot of work with 22nd century Romulus to inject colour and life into them, and I look forward to reading 'Bloodwing Voyages' when I get around to it, but I had hoped the TNG-Relaunch could do for Romulus what the DS9-Relaunch has done for Cardassia and give it sophisticated depth.
4) I think some of the novels should also continue the work started in DS9 to test the idea of the 24th century paradise. I don't mean that they should involve mass destruction, but just that our favourite characters continue to have their principles challenged. Ira Steven Behr was always very clear that his efforts on DS9 were to explore the paradise of the Federation and its limits. I think part of why DS9 is so compelling is that, through challenging the accepted precets of the Trek Universe it actually helps to strengthen them. It provides a depth of plausibility to the rest of the Trek Universe because we see the inevitable dark side of paradise.