This isn't an idea for a challenge mode, per-se, but I think it would be cool if, since we have all of these new unique items, if there was a way to earn them through challenges. Someone mentioned before the Call of Duty 4 level-up system, and haveing played that game I thought the challenge ideas were fantastic.
So, say you implement a set of Pistol, Shotgun (all three types counted), Chaingun, Plasma Weapons (Rifle and BFG), and Melee Weapon challenges. Each would have three levels of proficiency, based on kills, and at the max level (a rediculous amount of kills) you get access to a special version of the weapon at start. These weapons would be chosen from a list, and runs using those weapons wouldn't get saved to player.dat. Here's what I imagine they'd look like:
Pistol Mastery: Rank- Novice
- Kill 200 Former Humans with a Pistol
- Kill 100 Imps with a Pistol
Shotgun Mastery: Rank- Intermediate
- Kill 100 Hell Knights with a Shotgun
- Kill 50 Barons of Hell with a Shotgun
Chaingun Mastery: Rank- Expert
- Kill 50 Spider Masterminds (snerk) with a Chaingun
- Kill 50 CyberDemons with a Chaingun
Melee Mastery: Rank- Master
- You have access to the Power Sword, Daemon's Bane
Obviously some work would be needed to make it "balanced", but I think it'd be a fun reward for players who persevere.
*Puts on Flame-retardant suit*