My dream post sequel trilogy book, would be either a non-canon novel or comic that adapts Lucas's sequel trilogy concept that was dumped once Disney and Kathleen Kennedy took over. I would love to see just how different it was from what we ended up getting.
Why? What point would that serve? Well, we got "The Star Wars" comic adaptation so I could see this happening.
Mark Hamill's hints have been about it, stuff about Luke training Leia to become a full Jedi, something that wouldn't have been filmable anyway due to the long gap between films.
No, I'm not joking. EDIT: Since I read what's been oficially finalized about Leia, I've changed my stance to saying that Leia as a young women in a future movie should be played by a younger actress.
We finally have our answer. Carrie Fisher will appear in episode 9 as Princess Leia Organa, as it should be. They will be utilizing footage shot for The Force Awakens but never used. I say any chance to see unused Carrie Fisher as Leia footage is a good thing. https://www.starwars.com/news/star-wars-episode-ix-cast-announced
Of course there is - yet. They'll likely get back up with Rinzler and revive the book (and ones for TLJ and IX) only once Episode IX is done. As in-depth as his other books have been, there's every chance he'd have inadvertently spoiled (or just hinted at) plot points for TLJ and IX prematurely. Plus a warts-and-all portrayal of behind-the-scenes turmoil before the trilogy was even done could boomerang and impact that trilogy's completion. Given what an extreme secrecy-phobe Abrams is, I'm frankly surprised they were even starting down that road.