To be frank, there's been so many novels set during all of the series, I think we could pretty much do without any more unless there is a really pressing plot reason for the choice of timeframe. Have you given any thought to how packed with events those periods actually are ?
Admittedly that is even more true of TOS and they are still being written, but enough is enough - there's plenty of other eras / periods about which not much is currently known to fill in...
I don't know. I think there are a lot of books set within the series that are intended to just be standalone episode style novels, that's true. Lots of random extra-episode-style stories. But I think there are still a lot of open holes that could really have stories written into them - a la Crusher leaving, the rotating main Engineers, early Voyager, just like GalaxyClass1701 mentioned - about which we know very little. I think you could write like 7 books set in TNG's first season alone, just from dealing with all the strange awkwardness that was going on in the production then.
I think approaching each TV series the same way the Lost Era was approached - taking references and missed opportunities and turning them into stories - could result in a lot of neat in-series books still being published. (Sort of like what Distant Shores, Prophecy & Change, and The Sky's The Limit did.)
I do agree that I wouldn't want to see a lot more novels like the average numbered novel published, though. I'm seeing a lot of those coming up for TOS and I'm just so not interested.