Currently playing on randomiser, and have a few suggestions. Since it’s randomised, Elite 4, Gyms, and cult leaders can be either much easier or harder. A possible way maybe to make the main fights harder on the Hard difficulty is to make certain they have certain kinds of pokemon. This is already done for Megas in their teams, but perhaps it can work in legendaries as well, eg. Calreath’s team on hard being 1 random Mega, 1 random Legendary (replacing Manaphy), and 4 other randos.
This could make later parts of the game keep being challenging, as it’s possible for the player to find powerful legendaries or items early on.