Feel like the tools for editing novels are lackluster. Personally, I think there are multiple and easy applications that would benefit the quality of writing an author could produce.
For example, a 'readers preview' function that allows authors to highlight certain sentences or words and add a side note. It would help with organizing and remembering important details for the story. This would definitely increase the quality of writing on the site.
A word check would also help. What I mean is for repetitive use of the same vocabulary. If I select a word, it should output the number of times I used it and highlight its location in the chapter.
I think a poll system could be another great addition. Since authors here upload on a chapter by chapter basis, having a poll would increase a reader/author interactions.
These are just a few examples of many more possible changes to increase user friendliness for authors. :)