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A crossbreeding mechanic for one family.
Suppose there's a Pokemon with three different forms, each with different stat distributions and abilities (so each is viable) - one poison, one ice and one electric. Breeding one form with another will result in a "mule" - a separate species (also with three forms) that itself cannot reproduce (put it in the undiscovered group)
Electric + Poison = Electric/Poison
Electric + Ice = Electric/Ice
Poison + Ice = Poison/Ice
You'd basically get six pokemon for the price of two.