I recently played Ultimate Doom and Doom II: Hell on Earth, and I noticed that quite a few things that aren't in DoomRL that would be a great addition, as well as things that I thought would be cool to add to DoomRL.
Secret Doors
Some walls aren't exactly walls, but doors to closed off rooms. Getting into these rooms will be either helpful or harmful, much like certain levers.
Revenants
Apparently when a demon dies, they pick him up, dust him off, wire him some combat gear, and send him back into battle. No rest for the wicked, eh? You wish your missiles did what his can do.
Revenants are armored skeletons that can fire two missiles per attack, and can hold their own in melee. It took me about three direct missile hits to take out one of these creatures. Also, they can either fire unguided or homing missiles.
Spectres
Great. Just what you needed. An invisible (nearly) monster.
Spectres are practically the same as the Demons, except that they are partially invisible. Attacks against these bad boys, unless they are melee, have a 50% chance to miss.
Spider Mastermind
You guess the Arachnotrons had to come from somewhere. Hi, mom. She doesn't have a plasma gun, so thank heaven for small favors. Instead, she has a super-chaingun. Crap.
Equipped with a Jackhammer speed Chaingun, the Spider Mastermind is one of the most powerful creatures in the Doom series. She can take up to 18-20 hits from the Rocket Launcher, and deals around 26-28 points of damage with her chaingun per hit to Doomguy. She also constantly shoots until Doomguy moves out of her view, and then he can move back in to score more missile hits.
Baphomet, the Icon of Sin
Oremor Nhoj Em Llik Tsum Uoy Emag Eht Niw Ot!
You don't know what the hell it said, but you'd better kill it fast.
The Icon of Sin is a large wall consisting of a goathead with a hole in it's forehead where its 'brains' are located. It can spawn demon cubes which spawn about 1-2 demons (ranging from Imps to Arch-viles). The trick is to get on a platform and wait for the right time to launch a missile into its skull, rinse, wash, repeat. Hopefully you can kill him before enough Arch-viles spawn.
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Tower of Babel
The tallest tower in Hell, and rejected by God, there has never been a soul that has been able to climb to the top.
The Tower of Babel would be a secret level (much like Hell's Arena, etc) which consists of 20 levels, with no health bonuses or health globes. You'll be fighting harder and harder creatures, and at the top would be something really tough, like four cyberdemons at once or something. Your reward would be just as great as your struggle to the top.
IDDQD
ID Delta Q Delta. Degreelessmode on, rock and roll!
You should add a cheat code (much like the wizard/exploration mode of NetHack) where you become invunerable at the cost of losing your highscore/unlocking new modes. Of course, there would be a prompt that asks if you really want to turn it on.
Angel of Limits
All that junk is just weighing you down. Holding only two of every item sounds better to you, but with limited supplies, you'll have to choose your battles wisely...
Basically, you can only hold two weapons (including melee!), two stacks of any ammunition, two armors/boots, two items, and two mods. Not only that, monsters drop half as much equipment as normal.