Hi.
Minecraft is awesome. Congratulations!
An idea I had while playing and watching others play:
The game has these tendency for quite a number of players to use it as a creation tool for all kinds of machines. Or... since those machines are all virtual you could call them "programs".
Redstone (and recently noteblocks, for example) really makes people creative and their creations are often just... overwhelming.
So when I watched some redstone-creations on youtube (minecraft CPU

) I kind of got the impression that the way space (and time) are simulated really is beginning to be a restricting factor in the game. Of course, restrictions push creativity but when programming, could you imagine writing even slightly more complex program code without subroutines? You would soon end up writing thousands of lines of code for things that could be coded with two dozen lines and a few functions.
So here is my idea:
"Universe in Box" is a Block that can be constructed and given a name. (For example: "Bob".)
Inside that Block there is a parallel universe which is seamlessly connected with the universe "above" it. The block in which the parallel universe is contained exists both in the universe "above" and the parallel universe in which it marks the universe's "center".
So when something enters the "Universe in a Box" from the right (anything. A player, a monster, a Redstone-impulse...) it comes out next to the center (to its left) of the parallel universe inside the box. This also works backwards.

"Universe in a Box"es can be:
-
built (requiring some rather rare materials?) (when building a universe in a box, a new universe is created. The player must give that box/universe a name when creating it. (For example: "Bob".)
-
picked up and placed.
-
Destroyed. (which also deletes the entire universe contained in it)
-
Placed in parallel universes which enables "endless" hierarchies of universes in one another. Yeah.
So what do new parallel universes look like?
They could either be like a new strip of land or just an endless plain made of a material like dirt or sand. The sky could be a static night-sky or just *black* to keep the "real" world special.
Being able to dublicate existing universes might also be fun... but that would only make sense if people couldn't use it as a means to simply dublicate large amounts of rare materials... so I don't know. Maybe all "rare" materials inside parallel universes should simply be restricted from being copied and simply dont exist in the duplicate anymore. Also: Duplicate Universes would require to be given a new name upon creation...
What do you think?