I looked it up on Bulbapedia, and almost all of the trainers found there are Gym Leaders anyway, so I see your point. After all, the only "Different" trainers there are either Champions/Former Champions (Which might be considered similar to Gym Leaders anyway) and about 3 different trainers (Including Hugh but excluding Blue since he is Former Champion). So yeah, I can see your point.
The only Clubs that I think are suitable enough is a Rival Club or a Elite Four Pokemon Club, but yeah, I know I can only choose one.
Is it okay if I reserve a Rival Club?