Harder Angel of 100 would be a nice addittion to a new version. It is currently ranked as a Blade challange, but in reality it's quite easy. Once you are past level 35 or so, you are an unstopabble death incarnate: a level 10+ char with modded advanced weaponry, tons of armor, a stack of large meds and homing phase devices and some uniques to boot. And it gets easier as it goes deeper.
So, my idea is to spawn uniques on deeper levels. For example:
Cyberdemons: yeah, instead of simple knights and barons you'll have to face Cybies. Maybe on harder difficulties, multiple Cyberdemons may spawn. Player should be warned of their arrival, though.
Angels of Death: a powered up version of demons. Since you will be able to use ranged weapons on them, they won't upset the balance. I think.
JC: well, maybe it's too much. But levels 90+ on Nightmare...