I have long documents to annotate, so it would be nice if I can easily customise an outline - I suggest:
1. being able to create multiple layers and collapsable outline (pretty much self explanatory it is very important for users with long pdf. I NEED STRUCTURE!!)
2. swipe left/right on the side to bring out a sideview outline panel (best if semi-transparent background and on the side instead of a new window in the centre so I can reference the outline big picture while still reading the page)
3. being able to delete or edit the outline of imported pdf (most of the time when importing pdf with existing outline it gets all messed up and I can't delete them. either I ignore them or press the filter button everytime its so annoying)