Here is the idea:

A map or gamemode, where you can build on not only the land, but can also stick blocks to the other edges of the map - the wall of the map, and also the ceiling -, and when the block construction is connected to either of them, it would not fall down.
In short, all six sides of the play area would function as the ground.

It is not a very extreme gamemode, but it could be very funny and bring more space for impossible buildings and tactics with them.

For a second I thought it was an idea for us to have spiderman esque warfare, which would've been awesome.
Otherwise... Idk.

what about rotation where the center of gravity is on each wall so you could walk up walls :D

Or give us magnetism and Iron boots so that we can have the same experiance thier was in LoZ TP.

that would be bad ass.. (starts humming the spider man theme)

And then we could shoot webs :3.

OH MAN this would be the coolest game EVER if we could do that.