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Oil is the best candidate for a new type and here's why:
>Allows for new designs based on oily things that don't fit with the existing types, such as soap and paint.
>Is an easy concept for children to grasp, unlike Sound/Light/Cosmic.
>Fits a variety of existing Pokemon (Grimer, Octillery, Solosis, Goomy, Pyukumuku, Sinistea).
>Can logically beat Water, Steel and Fairy.
Only downside is that it would give Fire an unnecessary buff.