I'm champion of johto and kanto what am I supposed to do next?
You can take on the Elite Four again, they will be higher level than before.
Once you have defeated all 16 gyms you can go to Mt. Silver and take on Red. His pokemon are level 80-88 so be prepared.
After defeated Red go and talk to some of the key people in the game like Professor Oak, Mr. Pokemon, they will have surprises for you.
There are also a ton of legendaries in the games, like ohioscyther said. See this question: http://pokemondb.net/pokebase/763/what-are-all-rare-pokemon-you-catch-heartgold-and-soulsilver
I have to admit hg/ss does not promote after beating the game training much since wild pokemon are such low levels. I would first catch all of the legendaries in the game, and then work on your pokedex. Beat all of the tower tycoons at the battle frontier. Go on wifi battles against other people's teams, and then train to be the champion. I advise trying to go to a pokemon regional tournament coming up if you are close enough to one. I hope my advice helped you;)