I like the concept! It's sort of like Left4Dead's campaign mode but with a game master to control the enemy team.
What's to prevent the QM from putting all his monsters in the room right by where the players spawn, so they get swarmed and overwhelmed immediately before they can bundle up on weapons/ammo? It might be better if the QM gets a budget and a steadily escalating income to spend on monsters as the level progresses. That way the harder and harder monsters start appearing as the QM's income increases. And of course, monsters cannot spawn anywhere that is visible to a player (need to spawn around a corner etc).
Another option is to limit monster concentration based on valuable objects in the area. Areas around locked doors and items of power allow higher monster concentration than areas around the info_player_start or spots relatively sparse in items/doors.
A final idea is to mimic Left4Dead's versus mode. Instead of one player on the enemy team, there's 4-8 demon players vs 4 rangers. The computer controller handles weaker enemies like grunts, knights, dogs, etc. The demon players get to spawn as special demons like fiends, ogres, tarbabies, hell knights, shalraths, and shamblers. The frequency of each special demon spawn is based on how powerful it is -- ogres spawn regularly, shamblers spawn less commonly.
Another issue to consider is power levels. 4 players are an incredible force to behold -- just one is a serious threat. I can beat most levels in skill 3 without too much trouble, maybe dying once or twice, and I'm not that great of a player. Add 1-3 other players and it's a serious problem for the enemy team. Players can either stick together to combine their firepower, or they can split up to divide and conquer the level's challenges. Players can even circumvent entire swaths of the level with rocket jumping and other tricks, and they're extremely fast on their feet even without such tricks. Most Quake monsters, even shamblers, are not equipped to take on multiple players, even when the monsters come in swarms!
That means there needs to be some power balancing between players and demons: Players need to have lower defenses and offenses, and special demons need to have higher defenses and offenses, and probably increased speed.
Left4Dead had some clever methods for dealing with these challenges:
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Slow down the players: They should still be able to get across gaps etc, but somehow you've got to slow them down. Another option is:
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Speed up the enemies: Their movement and attack speeds should probably be significantly higher!
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Daze players on hits: L4D momentarily hampers a player's movement when his is injured, so that his movement speed drops significantly and slowly recovers over 2-3 seconds. This means that when a zombie runs up and whacks him, he has to turn around and deal with that zombie (and the others that are running up to him) or he's in big trouble. Quake's attacks are different in nature -- many monsters have ranged attacks, for one thing. Still, this should be doable with some minor adjustments.
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Make sticking together and working together a matter of life and death: In L4D, a player who runs off on his own is a dead player. Zombies will soon spawn somewhere nearby and then race in to overwhelm him. Players need to stick together to cover each others' backs, combine firepower, and save each other when things get bad. If they don't, they die.
If you don't want to be spawning swarms etc, and you don't want to trust that enemies will be able to challenge a lone player sufficiently, you can add another benefit to sticking together: As long as players are within a certain proximity and have a clear line of sight to each other, they have a bonus to defense and offense (say, 25% or 35%, maybe as high as 100%?). A player's percentage of that total bonus depends on how many players he is with. If he separates from the group, after 5-10 seconds his bonus disappears and he'll need to find his allies to get it back. If the group is cut in half, and each player can only see half his allies, his bonus is cut in half.
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No infighting or indecision for monsters: Monsters in Quake Coop are easily fooled: Two players trading shots on a shambler can easily take it down with mundane shotguns or even axes, because he keeps changing targets! Monsters need to pick a target and pursue it furiously, and should only switch targets when another is significantly better (dog is running after player A, who is 1000 units away, but rounds a corner and is right by player B. Consider changing targets, especially if shot by player B). Infighting should also be eliminated or greatly wilted to keep the monsters from getting too distracted.
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Change the nature of some monster attacks: Normal demons that players don't control should probably have fairly basic attacks, and ranged ones shouldn't spam too much since that can be too much for players (especially if those attacks are slowing them with a hit).
Player-controlled monsters should be effective, but not too powerful or spammable. For example, the ogre can use his chainsaw as often as he wants, but can only fire a grenade every 5 seconds or so (this cooldown time doesn't prevent him from using his chainsaw while he waits).
Finally, make sure enemy attacks are easy to use. The shambler's lightning attack should be very effective and should encourage players to be afraid of it, instead of only doing a bit of damage. It should also be easy enough to aim that the player doesn't easy waste it due to clunky controls or function.
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Ration player healing: Players are sometimes given ample opportunities to heal themselves, and other times are healing starved. This is a big problem in coop when you have 2-4 players getting hurt and needing heals. It sucks when a player with 95 health accidentally steps on the 25-health kit, which his dying ally needed. Medkits could possibly be replaced with an item they have to stand on top of to heal over time (maybe 10 seconds for a full heal), and no health is wasted. When the heal point runs out of juice, it deactivates. Also consider increasing and decreasing health kit heal amounts (and rate of healing) based on the difficulty level.
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Change the nature of some player weapons: This doesn't just mean powering down their weapons or powering up enemy defenses. Add knockback and stun to some of the player weapons, such as shotguns and grenades. If you're doing the enemies right, they'll need it, and this will add an extra element of strategy for their survival.
Those are my suggestions. I hope they're in line with what you're planning.

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