Have you done any attempts at multiple narrators telling a story?
Microsoft's Azure has a great tool for doing this but it's time consuming as you have to take all the text & match it to the narrator by hand. Open AI's last big demo kind of showed using voice chat to change narrator voices on the fly.
I think it would be awesome if you could submit a book, have a simple tool parse through & find all the speakers. Then let you sample how each one sounds with a brief description of what the person is like. Basically you get to have each voice do an audition & you pick your favorites. Then it goes through page by page generating audio based on the voices selected.
I'm not suggesting this feature for the app. I'm just throwing out this idea as one I've been thinking about. There have been a lot of books I've wanted to listen to but don't have time to sit down & read.