i just think the overworld is like that old idea of he earth bieng flat ontop of giant turtles, but without the turtles.
and the end is floating in the void under and u can fall onto it if u go into the void.
obiously u cant really do this but u asked 4 a theory, so...

You know, I kinda have the same theory, and sorta made a blog about it. I think the Nether and the End are inside the Overworld, many universes existing inside it. Just look up Hollow Earth Theory and you'll get a general idea of what I mean.

I made a HUGE mistake, said nether in the question instead of void. All I can say is oops!

I think the Nether is not a core, not a dimention, BUT A TIME. Think, a meteor struck the overworld turning all the dirt and stone into lava and brurrowing so deep that the celing caved in and created a large, open cavern-like place filled with lava. The pigmen are evolved animals that had to adapt to the heat and the ghasts/blazes are the souls of "past" Minecrafters that died in the explosion. So, when you create a nether portal, you are basically building a time portal into the far future. As for the bedrock celing with nothing above it, my best guess is that the atmosphere burnt up and everything above is just space.

Nether/void/end opionon: The Nether is the core of Minecraftia, hence the lava everywhere, fire, ect. The creatures there were adventurers, and animals that came there wondering why it was there, and to kills the legendary Nether Dragon, who later escaped through a hole in space and time and broke a link from the nether to the overworld and created an accidental  dimension called the end, and became known as the Enderdragon, but if you fall through the void, you don't simply go to the end, because the end is much further down and you die from lack of oxygen very fast , so you can't reach the end without using a portal which is made up of the void, and supplys you with oxygen on you're way down. Anytways, that is just my opinion. Real Story: Mojang created Minecraft. Perioid.