Hello,
From my understanding, the only pokemon that is not possible to breed for competitive use is Ditto. Which is very sad.
The reason would be that if you manage to breed them, it would break the way the other pokemons are breed (through the egg groups).
However, with the coming of gracideas and nature herbs for shaymin (and unown), it would be possible to do something similar with Ditto.
My suggestion is:
- Create items that work li