Doubt it, after the champion we will already have all Pokémon's available and have access to level 100, furthermore if they were to continue making Pokémon fangames I would imagine they would make an entirely new one from the beginning. I'd say the chances of this happening is around 0.1%. Maybe there will be some side quests in the post game that requires you to go to another region perhaps, but I doubt there will be a story there and I doubt you would battle the gym leaders etc.