I'll guess i am not the first one to ask for this... but I will do it anyway
Any type of advanced UI modification is much wanted! I think of LUA for an example.
The ability to modify size of windows, positions as well as content in game is a big benefit for all more advanced gamers as well as us creative interface authors. To be able to make custom in-game windows would also be a great feature. This would (among other things) be useful for information to end user and for configuration of skins and other game add-ons.
Advanced interface modification/scripting in combination with macros can greatly improve the "effectiveness" of roleplaying (macros for speaking, emotes and so on) as well as general game improvement.
I believe that this isn't a trivial matter and may take some time to improve.
Thanks for a very well made game!