I've compiled a small list of suggestions that would make the game much more playable/enjoyable, from a UI perspective:
Numerical cooldown counter ability overlays (with ability to turn on/off via settings).
A checklist (with checkboxes) for all usable equipment indicationg prereq's and having requirements that are already met checked. Including level requirement, usable by, skill requirements, etc.
Crosshair customization (oftentimes the crosshair gets lost in the clutter of a battle)
There is an issue with dragging gear where, when you replace a piece of gear on your character, the gear you're replacing sticks to the mouse cursor and you have to click somewhere to release it. Moving around gear in inventory and equipping gear should be more intuitive and fluid.
Being able to open more than two windows at a time (i.e. inventory, shop, and character sheet) would be nice. Perhaps introduce some sort of tab dock for windows?