Off the top of my head. None of these take system/scripting limitations into account (I do not know said limitations) nor do I expect them to ever be implemented:
- Zangetsu/Tensa Zangetsu
- Cardboard box from Metal Gear1
- VF Blade from Metal Gear Solid 2
- Templar shield (Google yields quite a few different designs for this one)
- Stone of Jordan (from Diablo II or III, does not matter)
- Alucard's Sword/Spear
- Burst Armor from the original SE provided it is not already implemented
- Skeleton "doll" to represent the most grinded mob from the original SE
- 4 star Dragon Ball
- Vital Star from DMC. Could act as a potion.
- Class "relic" from Borderlands series
1. Could be a helm that changes the wearer's sprite to that of a cardboard box.
Anyway, those are just a "few" ideas. I am sure I will have more later.
An addition: The Fork of Truth