I am not sure if this is the proper place for this but I have a couple things I think most people would appreciate in terms of UI customization:

1. UI Scale - The ability to "shrink" or "enlarge" your UI Interface (Or each individual aspect), allowing more/less of the game screen to show. This helps with those that need to see more of whats going on, or those with visual disabilities.

2. Rotate Action Bars - Allow each action bar to sit horizontally, or vertically. Not just the couple options you have left us.

3.Lock/Unlock Hotbar shortcut - Not sure if this is in the game but a shortcut to lock/unlock action bar to allow for quicker customization.

4.Toggle Quests - Allow us to toggle which quests show on the list.

These are the only ones I can think of so far...others can reply with further suggestions/comments.