I guess nobody knows, but launch time is coming and it would be good to write some mods.
There is one that might be very useful - saved skill sets, so we can have few skill selections defined and available with one click/shortcut (like Alt+1 to have skills for sneaking, Alt+2 for magic etc.).
Some people complain about UI being very ascetic,I like current style a lot, but we could give them much buttons around screens to click.
There might be addon to refresh food/potions buffs (well this will always be in MMOs) also.
Beside that I got no idea how modding of UI can be used more.