You know.....
I already mentioned I prefer my TrekLit set after the shows, but there is a certain type of novel I wouldn't mind.... There are plenty of episodes that leave you hanging. Plotpoints that make you go 'huh, I wouldn't mind if we get a follow-up to that', but we never did. And you could set those stories DURING the shows.
I know this is borderline plotsuggestion, but I'm gonna risk it..... Say, a novel that shows us what happened with the Malcorians and Mirasta Yale. Perhaps set during the last season of TNG, or between TNG and Generations. Stuff like that.
I could get behind this, Hollow Men is a great follow on to 'In The Pale Moonlight' (and even lead in to His Way).
Also, there are a number of novels which use elements/ideas/species from a specific episode (Takedown springs to mind), which in my opinion don't work so well as a specific sequel would.