Why don't I have a custom title by now?!
For a while I was skimming through this thread with my stubborn ways in reserve, hating everyone's ideas, but letting the thought sit in my mind I came up with some things that are doable with source ports but I wish was already in Doom.
I'm not really sure why id didn't think to have an instant death sector effect for really long pitfalls, or at least falling damage or something (but preferably death-only, without the 20 damage minor fractures like in Half-life, quake, hexen, etc.) That coulda really made maps like Downtown, Industrial Zone, The Chasm and The Living End pretty special.
Truthfully, a sector effect probably would have been abused pretty bad in custom maps, but it would really make height variation more prominent in people's Doom maps, and we'd have a successful return of things like narrow catwalks and jumping puzzles and stuff that seems to be non-existent in a lot of high-profile maps. There's something about landing in 20-damage blood and waiting to die that doesn't have the same thrill of splatting like an old tomato.
The only other things I could want are more aesthetic things, like i.e., a dreadful scream to follow up a long drop as you plummet hopelessly to your death (like in Hexen, if the above suggestion was included too) I'd also like a louder more agonizing scream when withstanding extreme amounts of damage, or taking damage when health is low, like in Duke Nukem 3D. Also visible damage on monsters, destructible scenery, water splashes, extra animation frames for the HUD weapons, mirrored death animations like in Retro Doom, shell and bullet case ejections, and a little more animated blood splats (even if they just fell in different directions instead of just straight down)