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Okay, so if you allow everyone to make custom Pokemon of their choice, everybody makes everything OP beyond belief. Like you show up to an OU match and the opponent has a Steel/Bug with 255 in every stat and Wonder Guard.
That's entirely subjective but I want to point out that if you want to do it your way, you can either run your own Pokémon Showdown client or make a Pet Mod (forum link) about new Pokémon/forms/items/moves etc, as long as there's a tangible concept between them (please read the rules!)