I think cowboys, farmers, clowns, firefighters, and rappers might make a good addition based on the fact that they never had these types of trainers before (except that cowboys were called 'riders' in Colosseum/XD).
If there were to be cowboys, I think the variety of Pokémon would be very limited, because horses are most often associated with cowboys, and there's only Ponyta/Rapidash. But they could always just give them whatever like that rider from XD who has Beautiflies and Dustoxes. Or they could just make very few of these trainers.
Farmers would normally carry farm animal-based Pokémon with them, such as Miltank, and Torchic if they allow it (I find that very few trainers have starter Pokémon). If you disagree with the idea of trainers owning Pokémon based on species, then I suppose any normal or ground type could work with farmers.
Clowns would have very colourful Pokémon.
Firefighters would have anything that beats fire. They should have more ground types than water, though, because there's too many water types and water type trainers (swimmers, fishermen, tube kids, some sailors, and sis & bros)
Rappers was just a random idea I had, and I know that it's very likely that there won't be any. But, just for fun, they would have steel types and/or very 'cool' (quotations are used because coolness is an opinionated matter) Pokémon.