It would be great if we were able to have a reference page (such as a character’s bio or location details) open while still having the writing window open. That way, instead of having to click around to access info and then having a chance to forget while navigating back to the writing area, all you have to do is glance over to the side, see what you need, and keep writing, uninterrupted.
ETA: Also, the ability to add words to the dictionary. I’m a fantasy author, so weird names come with the territory. I keep switching spell check off and on because all the names glowing makes it hard to spot actual errors. Weird to have a word processing software without spell check running all the time.