>>41691928For me, the best use of new types would be to retype overly used types. As the dex size gets larger, it becomes harder to differentiate mons of the same type, especially monotypes. Fairy did a good job of retyping Normal types that didn't have much going for them. I'd follow that outline.
Water and Psychic have a ton of similar monotypes. For Water, I'd add a new Fish type for all those generic mono water fishmons. Stuff like Seaking, Lumineon, Basculin, etc. would be retyped Fish. For Psychic, I'd split it into a Space type. Beheeyem, Lunatone, Solrock, and a bunch of those legends, etc.