So, people like things where it's easy to make recolors and OCs. That's why Silvally failed, all the colors are already taken. I suggest a Psychic/Fire type with 3 forms, an Infrared form, an Ultraviolet form, and a White Light form. This puts it in peoples heads to make their own interpretations based on their favorite color. As for design, it should appeal to both girls and boys, so go for a cute-but-not-too-cute similar to Pikachu or most of the first stage starters. And don't make it legendary, it has to be collectable. We want people arguing over which form is best as well as making their own versions. By the way, the psychic typing is so it's weak to Shadow Ball and Shadow Claw, as any light based pokemon should be. Maybe make the Ultraviolet special tank, the Infrared mixed offense, and the White Light one totally balanced. That way fan creations can be physical tank or mixed defense since it looks purposefully incomplete not having those two. All 3 get their own unique signature moves as well, each with a chance to raise a stat (Ultraviolet is SpD, Infrared is Atk, White Light is Speed) now the thought is in people's heads to make moves that can raise Def or SpA, or even Evasion or Accuracy if they're feeling creative.
We're trying to capture the same shit that made Eevee so popular, really.
