Simply make it realistic, dark and gritty. Have Pokemon battles be illegal, however they're huge entertainment, gambling attractions and have entire underground rings organized by the mafia, etc. The story would follow Ash, an orphan who grew up on the street, through some mysterious fluke of fate he saves the life of a pikachu. For some reason or another he decides he wants to make it big and enters the world of Pokemon battles. First he would start off in the small pathetic back alley fights, slowly working his way up, making a name for himself and meeting strange and interesting characters in his journey towards wealth and fame.