I'd say the ones you mentioned would make sense to go futa
Im sure I can come up with something for gens 6 and 7
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For those two I would probably say gen 7 you can do Silvally and gen 6 is like a break for you lmao they only add 6 legends/myths so you don't really need a futa, but if its one per you should probably do Volcanion even if the myths don't have any gender thing its male coded enough and also kinda reminds me of Heatran which feels like a fun angle to work with.