About suggestions:
I think it would be great if you'll make a secret level like exm9 in PC doom. There can be more than one secret tunnels of course. Also, secrets may be generated randomly and the can hide dangerous monsters... Just imagine, you open a secret and then you find your favorite bfg-cyberdemon, so cute...
About challenges, just for fun it'll be interesting to make Angel of [monster], and you restricted to monster characteristics/traits[i don't think about them], you play Vile and ressurect a monster, he fights for you. You play as zombie, equip armor and pistols[ just a dead doomer heh], or while you're playing demon you're in a permament berserk state+vampire and etc.
Angel of minimal damage, you do only min dmg, monsters do original/maximal.
Angel of chaos, you start from the river[hello cheaters], then you go through every non-random level, with 1 correction, basement levels are generated+some stuff.
Add a monster like Vile/Apostle... which can turn you into a chicken for a short period of time [they're dark mages huh]
Make a monster that looks like item[Nightmare hydras in G version] and they'll attack only when you near them[and info will show that it's an item].
Make a boss challenge, every lvl generates it's own boss with unique or exotic item.
Angel of morale, you kill enemy => you lose health.
Cursed levers, your equipment sticks to you, or smthg else. You push a lever then you die, poor rng.
Cursed challenge is good too, you start with cursed but good armor and weapons.
Invisible monsters should be visible under track map/int2 effect, while they are invisible you must have an opportunity to kill them, but, as far as i can remember, they are invincible while invisible.
More suggestions/bugs later.