It would be pretty slick if I could either click or hover over an inventory slot, say armor, and a pull-down window would appear allowing me to see everything I currently have that can fit i that spot.

An additional X that would toggle on/off items that are currently restricted due to level, race, alignment, etc.

The pull down window could be semi-customized via the UI settings or turned off completely:

Option A: The items are freely organized so the items you use most often are found near the top
Option B: Alphabetical order
Option C: Organized by Level 1-25 ascending or descending orders.
Option D: Disable pull down window.