Like the bible, the story didn't end when the last prophet rose to Heaven, and neither is Pokemon. If I could make suggestions to Nintendo, I would suggest they create a "Messiah" Pokemon who saved the world from the punishment of sin.
And here are what the generations and their worlds are based on:
Gen1: Nothing, the creator's imagination.
Gen2: Japanese legends.
Gen3: Greek style.
Gen4: Hebrew origins (Bible)
I think Nintendo should create, like what someone stated earlier, a medievil styled world and storyline. Or Chinese legends. Whatever it is, I think Nintendo is going to borrow the ideas for new Pokemon generations from cultures all over the world, so don't worry. Like everyone else says, it's not gonna end anytime soon.
PS: If they make a Pokemon MMORPG, it would hit big time!
- Hannah-Sama