Played Catan with my brother and father and introduced a sick homerule.
An Enclave is a union between two players that binds them to share 1/2 of the income of cities (not villages) between each other. To enter an Enclave, you must have a city. You can leave the Enclave, but cannot rejoin it afterwards.
Meaning that, say, two players have a city each which yields them 2 resources of X card. Establishing an Enclave creates a system where both players get income of 1 resource of X card, as it is split between them evenly.
That got me a win btw, >:D