The original plan for random treasure is what you see in game now, plus the following:
Items would rarely, in addition to the normal damage-dealing prefix they have, generate with a different prefix name and an additional special effect.
These would include some of the following:
- Meteoric
- Tempestous
- Infernal
- Glacial
- Erosive
- (and more!)
They're related to the damage type on the weapon. So a weapon with Flaming could sometimes instead appear as either Meteoric or Infernal.
Either a Shock or Screaming weapon could be Tempestous. There's enough rare prefixes to cover all of the prefixes that can appear on weapons.
The new versions of these effects are "procs", an effect that has a chance to occur on attack. They do things like, in the example of Meteoric, make a meteor fly in from the sky and smash into your enemies for AoE damage, dealing damage dice based on the level of your weapon, and forcing affected enemies to make a Reflex save of DC 20+Item Level or be knocked down.
There's also a lot of additional prefixes and suffixes for weapons, armor, and accessories that haven't been put into the treasure system.
Some of these include Humanoid Bane (combination of Elf, Dwarf, Human, Halfling and so on...), a new version of Bodyfeeder that deals a negative level to your target in addition to giving you temporary hit points, and adding some more staple effects like Blurry into accessories.
Many of the MotU multi-affix mutations have updated versions so they would be able to appear throughout more levels of gameplay.