When you start your world the game should save a copy of that seed. Then once you've progressed far enough you can go back to the beginning, but with everything in your original world. So you can go back and forward, and reclaim diamonds and treasures in the world that you already have. You could make special blocks like this:
But you would need the ETERNAL fuel source:
Of course those are buckets of lava not real lava. Each of them would be put into the Fuel intake portal frame thingy and then YOUR OFF! And once you've set up your house in both worlds, block by block your constantly zipping back and forth as you please.
ADVENTURE AWAITS!
(incase you didn't get that heres the fast version: theres a thing that makes you go back in time, but not really because you wont be there its just the world with a way back to what you had before, like a connection between two worlds so you can transfer items)











