The pot sounds like a great idea. The griefing potential is there, but it's no worse than FF.
How about...
Playing Blind - you don't see the little displays above the other players' heads.
Rescue - you can carry/throw another player at 10 blocks, during which you cannot use any weapon. Also, if someone gets hit, they immediately stop healing and have to be injected again, so you'll have to clear the area and get the wounded to safety first.
Dies The Fire - all player speech binds and voice chat are heard as ingame sounds, so you have to be within earshot of someone to hear what they're saying. Advantage: If someone says "Look out!" upon seeing a fleshpound and you haven't been mentally tracking where everyone is, you're not left guessing as to the pounder's location.
Divide/Conquer - the fastest zeds in a group will try to move into the middle of a visible group of players and try to dominate the area in between before heading straight for the players, forcing the players to split up while the slower zeds begin to flank or attack.
Diminishing Returns - the more kills you have, the less damage you deal, until other players begin overtaking you on the chart. (There could be an inverse effect with the fewer kills you do, which could lead to tactics where you sort of "bank" your "kill credits" to do some massive damage in the later waves.)
First Past The Post 1 - the zeds spawn constantly and you're always under full pressure, but they all run away and disappear once you kill enough of them to end the wave.
First Past The Post 2 - players respawn instantly during a wave, but if the zeds score six kills you're all brown bread. (unfortunately very, very griefable)
New Life - when you die, you respawn as or take control of an already spawned monster. (If there are no monsters left to spawn and no existing mosnters not already player-controlled, you spectate as usual.) You'll always be appropriate for that wave, but otherwise the game will try to spawn you as a random thing between stalker/gorefast and scrake. The mechanics are entirely the same and all interface is edited so that you cannot communicate to anyone and there is no way for anyone else to tell which zed is you. If you're killed as a monster, that's it and you spectate as normal and your killer gets the same money as they would for killing a normal monster of the same type. If you kill a (non-zombified) player, though, you get a lot of money - more than enough to cover both players' survival bonuses. As a monster you cannot damage another monster unless either of you is a bloat or husk (i.e., same infighting rules as the normal zeds).
Spawn Random - each player spawns at a random spot. Item spawn spots, patriarch spawn spots and trader doors can be used as reference points.
No Trader - the trader is gone from the game entirely. Between waves you can only regroup, heal, and scavenge. Upside: any weapon can potentially be spawned. With this mutator on, if you set the trader time to zero then the zed counter is hidden and you just get a continuous stream of monsters until the patriarch appears.
She Likes The Big WHAT!? - you lose the syringe, the MP7 altfire just switches between semi and auto, and all medics spawn as berserkers of equivalent level. Any weapon can potentially be spawned on the map. The trader is now the healer.
No Cutscene - when the patriarch appears, it doesn't interrupt the game. Upside: If he spawns nearby you can react. Downside: If he spawns far away you have no idea where he is.
Bunnyhopping Zeds - zeds bunnyhop as they move, making headshots much more difficult.
Mushroom Treatment - you are given no information about which wave it is, how long you've got for the trader, or how many zeds are left in the wave, and the trader arrow is hidden until the wave is finished. Trader lasts for two and a half minutes, however. Any zed can potentially spawn in any wave, though usually normal expectations are mostly met.
Market Forces - prices randomly fluctuate, even while you're in the shop. The more often an item is bought, the pricier it is on average until people stop buying it and it starts going down again.
While Supplies Last - the trader has a random inventory that is not the full selection of weapons. What she does have, both weapons and ammo as well as armour, are in limited amounts. (griefable)
VIP - each wave, one person suddenly becomes a level zero medic. If this person dies, everyone loses half their money. (possibly griefable)
FF Lite - you do 100% damage on allies but you can't kill them - a player's last hitpoint can only be taken away by a zed or their own explosives.
