I've started to scratch my own itch, trying to improve the audiobooks streaming experience. https://bookcamp.app I've got enough validation to think there's a market for it, especially in the Plex community. So for the MVP, the server acts only as a layer between Plex and the client. This allows me to build a great audiobook player and add features on top that Plex is not capable of.
1) note taking should not require having the book downloaded.
2) notes should be exportable
3) I’m constantly looking things up while I listen to non fiction. I think a visual stream of images tied to pronouns could be amazing. For example, when I listen to the history of ancient Egypt and it mentions Ramses II or all these tombs and tablets, it would be great to see thumbnails based on recently listened to content.
Anyway good luck!