What if theres a chance a pokemon can be born with a different secondary type depending on the parents or a very very small chance in the wild? there would also be a list of which moves each secondary type could learn like
Ghost type has a specific list
Water has a specific list etc

and the type could change the pokemons color hue (so they don't have to make individual textures for each type)

It would be pretty cool, kinda like how real life dna works and mutations