I liked Angel of Silence. The idea scares the hell out of me (reminded me of how much I count on it), but that's a good thing :)
Now, a suggestion of my own. It's more a mod than it is a challenge, considering the slight alteration to storyline, and balance-risky pros and cons. I'm writing this more out of amusement.
Angel of Hell
UAC? Who the hell is that? The closest you got to being a space marine was beating one senseless in a bar fight. You're here on a psychopathic joyride - think of a toned-down, human version of Lobo. Indeed, you're the very worst of the infamous Hell's Angels.
Pros: you're mean - damn mean. You start with a +2 to each damage and armor and 20 more hit points. Additionally, you cause 6 bonus damage with the chainsaw, or any other brutal melee weapon (therefore, LS not included - cheesy mythical glowing pansy-weapon that it is. Neither is the measly knife. If only you had a crowbar...).
Cons: you're also completely lacking in training, or meaningful purpose, resulting in a -3 tohit. Well, not quite without purpose - you're wholly intent on bashing as many demon skulls in as possible (as a token of sheer badassness), all the while enjoying this site-seeing parade through the Abyss. You must kill any demon you face, so once you see one, you can't voluntarily let it leave your sight until it's dead, dead, dead. You also can't avoid any special level once you've seen the stairs to it - you can leave it for a while, but that becomes the only way down.
Last but not least, there is one thing you must always care for above all else - if you lose you're bike, which appears wherever you enter a new level, you collapse in tears, and give in to eternal grief. Boo-fucking-hoo. Naturally, all of hell's minions gravitate towards this awesome beacon of chaos and destruction.