Basically which type do you think would've fitted in each gen? For me it'd be
>Gen 3
Flying, since it's a very "summery" and laidback region with lots of flying types
>Gen 5
Bug and Dragon, it'd tie in nicely with all the cool bugs and all the dragon lore
>Gen 7
Poison, because of the "exotic" theming of its dex
>Gen 8
Ground and Steel, since Galar is a very "tradition vs modernity" region