How about starting with basic UI feature integration that the popular mods offer. Some of these should be included in the vanilla client.
for example...
-secondary hotbars (icon locking options,vertical or horizontal options)
-Movable ui (movable, resizable,scalable)
-Resizable map (the display of the map area at least)
-chat text size options
-option to toggle show memorized nameplates ( persistant name plates option )
Personally i find the default client would greatly benefit from the basic functionality these things provide. It would go a long way in hooking potential long term players who may generate revenue in buying silver etc, if the interface and ui wasnt so poor.
Trying to actually play and utilize chat is incredibly abyssmal. As the chat window covers the hotbar area, and playing with the chat window closed I find it hard to read the text or catch it in time to read it. Some shine and polish in this area would be good for the game overall and improve the experience of newer players, improving word of mouth about the state of the game encouraging more players etc etc.
If there are any other functional standard UI features that would improve the standard client please add them in your response.