>>37586180I'd love to make it an even 20 but I don't really know what to add.
Sound is an obvious candidate given that it easily has the most Pokemon, with at least a couple from every gen, and moves to retype (even more than Fairy) and is thematically distinct from all the other types, but I'm not sure what other type would be good to get to that 20.
Light and Cosmic are popular choices, but neither have very many Pokemon to retype, and unlike Sound which is mainly tied into Normal a lot of the potential candidates already have non-Normal dual types that would make them unlikely to be retyped, like Lanturn, Solrock, and Palkia. Cosmic also has the issue of it becoming by far the most nebulous (pun intended) type, with no real logic to back up its matchups. Magic is another popular fan typing but it's even worse than Light and Cosmic for similar reasons.
Are there any other potential 20th types that would work well? Light seems like the best out of Cosmic/Light/Magic but I still don't think it fits in nearly as well as Sound or the other types do.