Apologies for previous lengthy post, anyway, back on topic, the addon's I'd like to see are:

Damage/Healing meters (if that wasn't already obvious lol)
Threat meter
UI skins (Default looks nice but after a while it's nice to be able to change)
Raid UI (similar to Grid)
Customizable cast bar
Chat addon (where I could change the font/have timestamps/have ability to copy&paste)