We could find a world that has been left abandoned and become inhabited by a new species. We may finally get a chance to come in contact with the Forerunners themselves or find an old familiar enemy that has taken control of the advanced technology and is now even more powerful than before. I can imagine an environment that has amazing futuristic structures that both housed the creators and allowed them to manufacture all means of weapons, vehicles and ships. There would have to been an extensive network power, mechanical and waste tunnels to support such a large creation. There would have to be research, command and control, education facilities, etc. I also hope there will be natural landscapes that have rivers or large bodies of water, forest areas and even underground caves and waterways. Could it be possible this massive creation was built around a small planet or moon?
I am extremely excited that the Halo 4 game will at least start, if not be fully played, in a completely new and alien environment.