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After seeing some suggestions, I think a way to do it has to also have a weakness built in
if it's just 100ATK+100SPA = 150Mixed, or "whatever would do the most damage applies" you're just going to make every non-mixed attacker irrelevant for the most part
How's this: after a move, the next move of the opposite type will be boosted, while the same type will be weaker
this way a pokemon like Gallade can bring both special and physical, and after a turn of mediocre damage will alternate between being special and physical each turn, which allows for counterplay
the issue is that it would have to either be from specialized moves from each type, or from an ability, you definitely can't build it into the game by default otherwise it'd just break every specialized pokemon
