Played with 3 friends on L4D (one of them doesn't have L4D2 so we play L4d instead), and we thought the game was too easy. So we set up a game and made these rules.
1: Two players are restricted to melee shove, but can use grenades, pills, and medkits
2: One player cannot use tier 1 weapons, but may use pistols until they obtain tier 2 weapons. He/she cannot use pills, grenades, or medkits. He/she is also not allowed to use melee shove.
3: One player is restricted to tier 1 weapons, but may use melee shove and pills. He/she is also the only one who is allowed to use the minigun (might not effect you depending on strategy).
4: For added difficulty you can also make certain people vital to progress. One example is designating someone the official door opener, them being the only ones allowed to open doors (almost as if all the doors have locks and only one person has a sort of skeleton key), or start events. This would make them almost like a VIP, someone who if lost forces you to restart the level.
This is insanely hard, but quite fun. It's more practical on lower difficulties, but for maximum difficulty jack it up to expert.