Urmm...
Though the wild Pokémon below Blackthorn are weak, the trainers have some though Pokemon (I think, I haven't played Crystal in years - but if it's anything like Soul Silver then I'm right).
There are plenty of challenging trainers for your levels on the way to the Pokémon League.
Other than that I'm stumped. I'm currently exploring Kanto on my Soul Silver, so I've completely forgotten about Johto! XD
