Also, I don't remember where I saw it on here, but I really like the Angel of Imp challenge. Maybe an evolution every few levels a la Pokemon or Zerg into a higher demonic form, with more power and even (gasp!) the ability to use items! Even if one were an imp the whole time, steadily gaining in power yet uable to pickup items or use weapons, that would be a blast. Berserk packs, health globes, supercharge, etc. would still work. Remove wall-destruction from (your)higher demon's energy attack, so you still need a rocket launcher to get through The Wall.