The crystal Onix was a very interesting idea, possibly a foreshadowing either shiny Pokemon or Steelix (which is also weak against Fire). I think it would be cool to have a crystal Onix in the games. It would be more tricky to pull off than Spiky-Eared Pichu though, seeing as you'd have to find a way to make it weak to Fire and immune to Water, while keeping the core characteristics that make it Onix