[Markimedes]

More of a game variant than a house rule. It's for 2 people, which is often the number of people I have available for a game.

First off, each player plays as two munchkins.
Each player chooses a color and takes both the male and female piece. Each player must now select a sex, and write that sex down somewhere secretly (I write it on the bottom of my foot, but I am weird). The munchkin of that sex is the only munchkin that player controls that can win the game. If the munchkin of the opposite sex wins the game, then the player controlling that munchkin loses.

Players cannot trade items between their two munchkins.
The change sex curse changes the sex of both munchkins under the player's control.

Otherwise, the game is played normally, with players taking a turn for each of their munchkins. Might need some more ironing out, but it's pretty fun.

Makes for some interesting play styles.. some things I've noted:
Since which sex will win is a secret to the other player, you are incentivized to try to disguise it. If the munchkin you are favoring becomes obvious, your opponent may concentrate fire on that munchkin with two munchkins. (Or even try to get the other munchkin to level 10, losing the game for that player!)
You can give charity to your other munchkin if they happen to be the lowest level, so it makes sense to try to keep one of your munchkins in that position.
You can use the change sex curse in order to make your opponent lose, or yourself win.
Probably other things too.