I would love to see some more interactive fiction (like Twine) possible. Aside that though, I think formatting to me, seems constantly poor. I think you'd have a really hard time rendering something like Hope Mirles' Paris (you might need to google it!) in ebook form well.
The main formats are plenty powerful with the formatting they support. The bigger issue is that ebooks need to render on a pretty big variety of devices, and users can override typefaces and font size. So short of using a PDF to override those choices, there's not a ton that an improved format could accomplish.
Also, ebooks do support links (I see them most commonly used for linking to chapters from the table of contents, or linking to appendix notes), so you could make interactive fiction if you wanted.
