Okay, first of all I'd like to say "hello" to everyone here (as it is, in fact, my first post here). And I'd also like to say that I find DoomRL to be a great game, Kornel. A little bit too random at times, but that's the charm of being subjected to the whims of RNG.
Now with that beneath us, and with me having gotten through the entire thread (that was tiresome, but some of the ideas were pretty amusing), I'll write down my ideas of possible exotic/unique items. They'll probably be just as bad as almost all first-time attempts are, but maybe you'll find some of them interesting.
Tree Cutter (Exotic)
Damage: (1d5)x10 [range: 10-50, average: 30] bullet damage
Alternate fire: Chain fire
Clip size: 80
Accuracy: -1
Fire time: 1.2 second
Reload time: 4.0 seconds
Ammunition: 10mm
Description: A weapon that emphasizes quantity over quality. Burns through your ammo cache like a chainsaw through trees.
Pickup quote: It's time to deforest hell.
Other effects: When used with Ammochain, consumes 2 ammo per volley (possibly stacking to 4 if an assault rifle assembly is grafted onto it).
My reasoning for that one is pretty simple. Other than the minigun (1 item), there are no unique/exotic chainguns in the game, so people who want to create an Ammochain-based build have to switch to plasma rifles pretty quickly (which isn't exactly bad, but that means that, most of the time, there's only one weapon for them to choose from). The only concern is the demolition ammo assembly, since when applied to most weapons, it reduces their damage to 3d2, and that means the ridiculous amount of shots will make this weapon superior (but then again, the -1 accuracy bonus will be a curse working towards balancing it, and taking Ammochain [which is pretty much a must for any characters using this weapon] precludes taking Eagle Eye, which means this weapon wouldn't be a fix-it-all item).
BFG 3000 (Unique)
Damage: 7d10 [range: 7-70, average: 38.5] fire damage, radius 7
Alternate fire: Point-blank detonation for 5d7 [range: 5-35, average: 20] damage with radius 3, uses one rocket.
Clip size: 5
Accuracy: +5
Fire time: 1.4 second
Reload time: 3.5 seconds
Ammunition: Rockets
Description: An old-forgotten ancestor of BFG 9000. Don't blow your hands off.
Pickup quote: Ahh, the good old times...
Other effects: Uses 4 rockets per shot; can't be modded, goes into your hands as soon as you get it (just like BFG 9000).
Yes, this weapon would be on the level of BFG 9000. But then again, the BFG9k itself can be modded further, and this weapon can't (because I know one nano pack would make it utterly broken in terms of power); that alone makes it slightly worse in comparison, and it becomes even worse if you factor in the fact that, without Fireangel, its explosion radius (and the alternate fire) makes it very easy to accidentally kill oneself. The 4-rockets-per-shot thing is nothing one could ignore, but it's a decent weapon against anything if you have a lot of spare rockets and Fireangel.