Hmm one thing I'd be interested in but I'm not sure how the functionality would work and whether you'd be able/allowed to do this in WoW.
In Neverwinter Nights they had these cool radial menus that you could bring up. You've already got circular bars...
Could it be possible to hide a bar and only have it come up when you hit another button, then fade when you click on one of the buttons on that bar?
What I'm thinking of is say I'm playing my lock. I only have one button for "Curse" and one for "Demon" and then I've got all my shadowbolts and stuff out like normal. The reason I only want one curse and one demon button is cuz I'll only have one out at a time. If I click on "Curses" it suddenly pops up a radial menubar of all my curses so I can choose one to zap my target with. As soon as I click the curse, the button dissappears.
Really, I think this would be way useful for a palladin's seals, for example.