The title is pretty self-explanitory, a planet that is a cube. Everything on the planet would also be cubes, squares, etc. But the catch is, it's an entirely artificial planet, that someone somewhere is making (like with a computer). So some enemies could be a pixel, a voxel, and maybe a mouse that spawns them.
I like it....MAYBE ITS A VR WORLD!! like, as a quest (or not) you go into a computer and this is the world. its all just virtual reality!
Improbably built worlds such as sky islands and cubes appeal to me for some reason, they're quite entertaining. Or rather, the thought of people sailing off the ninety degree edges to their doom is entertaining.... ....I think I have a problem. On a serious note, creatures that live on a cube world don't necessarily have to be boxes themselves, I think it would make more sense to have critters designed for surviving in such an environment. For example, I would think the ability to dig or travel across the surface quickly would be important, seeing as how there are large, flat surfaces to traverse. Perhaps a creature that makes the terrain less flat would be around as well, such as with the digging example: some sort of mole that makes hill barrows for their homes. There could also be some sort of vegetation in the form of roots and vines on the sides, which would give birds a precarious nesting ground as well. Basically you have to think outside the box (yes even I facepalmed at that.)
The way gravity works could also alter depending on the surface, such as when you reach the end of the planet, you would just continue walking down the wall and then upside down when you got there and your screen would rotate with it. It would require people to mark their places so that they wouldn't become disoriented while running around inside of the world and constantly changing direction maybe. Or perhaps you would just be constantly drawn towards the core of the world which would be the main piece that allows for the rest of the planet to be changed via the digital way that was stated in the original post. And then when you get to the core which would be guarded quite well by defenses and creatures you could take a part of the core which would corrupt the data of the world and it would begin to delete some parts (but not the entire thing, just random parts) and spawn corrupted data monsters that are more powerful and aggressive than the creatures before. The piece of the core would then be used in either crafting, or contain coordinates to the planet or whatever that the mastermind was controlling the cube world from so you could hunt him down or recruit him for some sort of use.
This sounds like some class-A Super Mario Galaxy shit right here... talkin bout cube planets and different gravity and stuff...