As you asked what could this 3D novel make it the best as it can be:
The same things that make great 2D visual novels or novels in general: excellent writing or/and an exciting story. That is in my opionen the most important thing. Visuals are the second. If it looks nice and supports the story with their asthetics it is perfect. This demo of yours has till now few of both. Also if you do use 3D, it woud be nice to use the full potential of it. Free movement and/or fitting animations. Maybe like a exploration game. Moving around a room, get hints, talk there with people. I think you can learn from detective games like the sherlock holmes games or other adventure games.
If you don't use 3D movement or 3D anmation then you don't need 3D to tell a story.