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Magma could easily utilize more grass Pokemon since their idea is that more land equals more living space for animals, so plants should naturally be able to expand as well. Fire and Rock types fit pretty well by proxy as well.
Aqua can just as easily use Ice types, but that's where the list of ocean types end, but given the stupid amount of water Pokemon in the Hoenn region, that alone gives them plenty of variety.
Generic Pokemon like Vigoroth, Whismur line, Zangoose&Seviper etc could also make cool picks. Personally never understood why ORAS had them use pollution species like Koffing and Grimer since that seems counterproductive to their environmental goal, so imo those can be disregarded.
