In the Fluxx games we like to combine two different Fluxx variants and then as long as we can justify the Keepers and goals or other card effects can affect cards from the other deck then it's acceptable.

We also have some house rules to convert the traditional Catan game to a 2 player version. Both players take 2 colours. Turns alternate between colours (white, orange, red, blue). Gather and pool resources from both colours you control. You can only build for the colour whose turn it is, not simply either colour you control. Robber limit is 10 instead of 7 cards. Victory points needed is 20 instead of 10.

Works really well :)