Topic: UI features
I think it might be nice if you could dock/undock windows to the UI. For example, have it where you can undock the chat window and expand it to be most of the screen if you're more interested in chatting at the moment. Or, undock the inventory so that you can look at what you have while chatting (perhaps you're telling someone the stats on the cool new hunting knife you found). Also, it might be nice at times to be able to dock a skill window out of the way of the screen so that you can still access it but can still see the full game screen (perhaps you're blacksmithing but want to keep an eye out for monsters and other players or something).
One last suggestion on this topic, can you make it so that when in windowed mode, you can expand the window and move windows into it? Would be handy in conjunction with a dock/undock option to undock and expand the chat and put it in a large area to the side of the game screen.
Oh.. just to clarify, I don't mean spawning a new window within windows, just a new "toolbox" style menu within the game (similar to what the skills and mail system do).