For computers- side bar with menu options and/ or pages
The ability to have multiple pages at once
Sounds and animations- if you can do it, keyboard haptics and/or a cash sound for dono
Page groups (maybe call them chapters/ books) to compile related works
Subtle Button animation, could be a change in color or a drop shadow or whatever, just to indicate when things are pressed
A repost page to show off pages you like to other users or some other way to add a more direct social element beyond monetization
Ability to highlight a section/ click on the numbers you want to select on a page and then right click on desktop/ long press or double tap on mobile to immediately respond with that as the quoted section (would be good especially with the ability to open multiple pages)
Page tags- functioning like a searchable category/ hashtag so if a topic interests you you can find it under a certain tag to read pages in that topic
Deleted/ recently deleted section
Autosave for long notes just in case they accidentally delete