Gastrodon and Shellos have already been done. If you want to learn how to make sprites, we can help you. Starting with recoloring shinies is a good idea because it is easier than making sprites from scratch. If you want to help us, start by recoloring something we haven't done yet, such as Skuntank. I recommend you download a free paint tool such as GIMP to make the sprites, then upload them (no matter how bad they are) here and I'll help you out