Eh, Cheren doesn't count, even though I kind of liked him. I really wish trainers would level up with you, like Cynthia would keep getting her levels raised until she reached 90 or something. That's what you call a real battle. Definitely Champion-worthy!
I think it would be awesome if your player character officially became Champion after winning and then you could accept battles from random trainers, like rematching Gym Battles in HGSS.