It seems to me that many of these ideas will have too much in the way of changing player strategy when found, by becoming too worthwhile to pass up. I wouldn't like to see the game control the player that way.
IMO, uniques should have clear disadvantages to counterbalance their advantages. Not enough so to make them no better, but more confusing, than normal weapons - just enough to keep them a consideration rather than a requirement. This'll also add to the "interesting" factor.
Now, for a few of my own ideas:
Name: Neutron shells
Base item: Rocket X10
Abilities: very high damage (X2-3 normal); doesn't harm walls, doors, barrels or items (though radius isn't diminished by walls, either). Both traits are counterbalanced by themselves - high damage includes player if in radius, and by not harming objects it can't shape the battlefield.
Name:Ol' Painless (minigun from Predator, IIRC)
Base item: Chaingun
Abilities: 12-shot (though same speed as CG), 120 clip capacity, 2d2 damage, very long reload (X2-3 normal for CG), recoil (1-3 spaces). Maybe double-handed (blocks secondary slot)? Maybe has 1 turn "warm up"?
Name: Shine On
Base item: Plasma rifle
Abilities: acts as SG for purposes of targeting (ie AOE weapon with 100% hit ratio and damage dissipation by range), but as PR for purposes of damage, clip capacity and number of shots fired.
Name: Faust
Base item: Combat SG
Abilities: Double damage. Doesn't need normal ammo. Reloads by causing player 1-3 points of damage per "shell". Maybe leaves taunting line on mortem if ever used (styled as "soled your soul to defeat Cybie")?