Munchkin with 7-12 players
In addition to getting more cards for the deck, have more people go at the same time.

This concept works for games of 8, 9, 10, and 12 players. Games of 7 and 11 present a prime problem.

Plan. Have two or three players kick open the door at the same time. Each takes a card off the top of the Door pile, but keeps it face down in front of them. Then, they kick it open simultaneously, and commence the fighting. 

The active players should be equally separated around the table. In their seating positions, 1 through X, the bolded seats are active. As an example, consider 8 and 9:
8: 1 2 3 4 5 6 7 8 divisible by 2
9: 1 2 3 4 5 6 7 8 9  divisible by 3  

A game of 10 players would be similar to one by 8, while 12 could follow either. 

An additional level of complexity could be added by limiting the range a player has. For a game with 2 active players, an active player and those sitting right next to him can't hamper or aid the other active player. In a game with 3 active players, the non-active players can only target the active player immediately to their left and right, but not the far player.