I found a game yesterday called "Minetest", it's an opensource clone of Minecraft coded in C++.

A Minecraft game would be pretty fun on the xbox, here's the source code if anyone wants to take a look:

https://bitbucket.org/celeron55/minetest/src

Gameplay/overview:
http://www.youtube.com/watch?v=ZTRGCi3qrEo

Looks like it uses irrLicht in some way, which has the potential to be ported to the Xbox, or so the engine developer says.

That'd be great. But who knows if it'll ever happen.