Realistic encumbrance. No more carrying 9 weapons at once, bitches! If you pick up too many guns you won't be able to move, so you have to reload an earlier save and NOT pick it up. This should add immersion.

As well as WASD to run, Q and E should control the movement of your left and right legs, which you must tap perfectly in sync with the WASD motion. Fail to do this correctly and you trip and fall down like the little girly-man that you are. This too should make the game more realistic and satisfying. 

Firing a rocket launcher should require the assistance of a co-op buddy. Otherwise you lose half your health from the kickback.

Also, rockets should blow up anything. Like doors. 

So basically Q1WOP? 

Make a whole level of swing gates, the player has no weapons and just has to crush enemies with the gates.

Something like this:
https://www.youtube.com/watch?v=0jS2j97wEGY 

That actually sounds like it'd work on Occulus with the Hydra control.

I wonder how long it'd take before the motion sickness set in. 

sure why not 

AD already added footsteps for monsters...it was creepy too. 

Wasn't it the Mission Pack 1 that had those horns that summoned temporary allies? Make another kind of temporary ally you can summon to fight for you briefly.

Or create a Conflict power like in Nethack, which makes all monsters fight each other for the duration. Decide how this interacts with monster infighting. It is probably quite powerful and should be used sparely.

Create new weapons with magic or alien tech theme. You could take ideas from Blood, Heretic, Ziggurat or wherever. You can have things like homing, turrets, slow/freeze effect on targets, and weapons that drain your hp when used or that give you hp when they damage enemies, but are in turn very weak.

Improve the monsters' ability to patrol areas. Make them investigate sounds and become alert if they see fighting in a distance (or a mysterious rain of gibs).

Implement bots you can control remotely and see from their cameras. Create puzzles where you have to operate bots to move stuff or open doors.

Finally, can you implement an inventory system with QuakeC? This would let the player save the power-ups for critical fights, or more likely save them until too late and never get to use them :) 

A monster attack that reverses the player controls.

A QuakeC-based API to script Quicktime events. I can't believe Quake doesn't have these yet. You could even brand them "QuakeTime Events", which would sound fantastic on a sizzle reel.

If all you purist guys insist on having pixelated textures, then at least put them to work by making the whole environment destructable / buildable, like in Minecraft. Missed opportunity IMO.

Proper story / cutscenes / anime between maps. Again, feels kinda bland without it. Like, ok I'm going to the Necropolis, but why am I going? What's my motivation? What are the stakes? Come on, this is really elementary game design stuff, guys.

When player dies, add a QTE to give him a chance to respawn where he died and carry on playing. Would make death more dynamic and interesting. The death mechanics are really bland and dated in this game currently. 

 

which one of the regulars is pepelord86

he's not even a rare pepe 

Rocket League in Quake, with big ball, guns and jump modifiers. 

...
A big fiend, you have no weapons, you need to lure him into the goals without dying. 

It will be slow as fuck... 

 

RAGE did that. Don't be like RAGE. 

you could call it cowardball 

I think you should be able to do most of this idea without modding. 

A new skill level above nightmare, called "elite", which is the same as nightmare except that monsters can damage a godmoded player, and see a notargeted player.

As well as killstreaks etc., also have non-combat related acheivements, such as achievement for when player runs 100 miles in total, or turns a total of 720,000 degrees.

Player can level up different parts of his weapons and gear, for example level up your holsters to make the guns faster to equip and stow away.

To work around the maximum ammo limits, make it so the player can carry more than one of each type of gun, so e.g. if you run out of rockets on your first rocket launcher, you can switch to your second rocket launcher, and so on.

Maybe make it so player can only jump on context-sensitive chasms and things because the jump noise sounds ridiculous when you keep hearing it all the time. 

Make a less intrusive jump noise like a quick boot tap sound.

In my younger days I created a coop mod where enemies gave you points and a sprite would float up telling you how much, then a bprint would let you know how many points you got and also your tunning total. An impulse would print your stats, current score, kills, deaths, etc. I created a couple wierd maps with a ton of spawning waves of enemies. It was a poor map, but the scoring was kinda fun...gibbing enemies gave you double score. 

 

here's the jump and landing sounds i put in id1:
shoresofnis.com/temp/player_jump.zip 

hrmmm
http://shoresofnis.com/temp/player_jump.zip 

I made another silly little mod in my younger days called Tower of Whispering Gold. I never released it for pride reasons (same for mod mentioned above.) Anyways I removed all weapons and enemies and made it where the goal was to collect all the rings (using the invis ring model) in each level so the exit would open. It was about exploration and such. Quad made you run faster, I had ladders, All the powerups did different things like negate fall damage. I made it for my little cousin to run around in. 

It had lots of traps...buzzsaws, shooters and the like. It wasall about puzzles and exploring