More TOS. Specifically what drove Spock to Kolinahr and why Kirk left for a desk job. I always felt the novels did a poor job when dealing with these issues.
I shockingly agree with you on that, actually. End times must be nigh.
Especially on Kirk. He was in his early 40s when he made admiral, and still actually could be said to still be in his prime as a Starfleet officer. I'd like to know if it was ambition he later regretted that motivated him to accept the admiralty, or maybe just the Powers that Be promoting a space cowboy to get him out of the way and keep Kirk under their thumb. It just seems so out of character for Kirk to go that route, and I would like to know why he did it. As for Spock, he always wanted to rid himself of his human half, so it is more expected, but I think it would have been an interesting story to follow. And, i think the fact Spock later embraced his human side to a degree post-death & resurrection, that I think his embracing his human half would have made a nice bookend to his earlier quest to rid himself of emotion.
Here are other things I think might have been cool:
1. Earth Romulan Wars, either during Enterprise IN LIEU of the Xindi mission, or on a season 5, or a Post-Enterprise Miniseries, similar to how Alien Nation returned as a miniseries, after the show's cancellation. I always thought that was a missed opportunity. The Xindi arc had a lot of good episodes, and the Xindi themselves were interesting, but I would have liked to see the Romulan war instead.
2. I would have liked to see a better send off to Tom Riker, than in "Defiant." He deserved at least one more episode, IE rescue, or chance at redemption, or something. They set it up like Kira was going to rescue him, but it's never mentioned again.
3. A better send off for Kes, than Fury. Really, I think Kes should have been seen as a semi regular, at least once a season, each time growing more ascended\evolved, similar to how Danial Jackson was featured in SG1 season 6, with Voyager season 7 finale depicting her as the "new caretaker." I would have liked to see her involved somehow in the return of Voyager to the AQ, and the escape\defeat of the Borg.
4. I would have liked some follow up with Spock's unification movement. I suppose its moot now since Romulus is gone in the Prime timeline, but that was one story I wanted to see more of, or at least hear about in a scene, even in a couple throwaway lines.
5. I know this is more a nerdy fanboy wish, but I wish the Constitution class appeared either in TNG or DS9. I get that the official reason is they didn't want the Constitution class on TNG while they were still making TOS movies, but during the Dominion War, since the Mirandas were still around, it would have been nice to see some Connies dragged out of mothball and put on the front line.