Good luck with your project
A screenshot ability could be a handy feature when trying to answer forum questions.
If you are able to safely
update the sqlite database from the device UI then a good candidate may be the content_settings table which holds font-size, page margins and line spacing per book. Although the standard font setting sliders allow many values, they have not been particularly well chosen by Kobo (IMO) e.g.
- font-sizes - not fine graded enough in the mid-range. Jumps of 1pt would be preferable.
- line spacing - current minimum is 1.3, which is (IMO) too big for many fonts.
- page left/right margins - again not fine graded enough. Current values (in points) are 0, 3, 6, 9, 12, 15. Again, jumps of 1pt would be preferable.
Speaking purely for myself, I have no interest in web options, but there will be many who do.