I just want to say that this is all speculation on my part and none of this is confirmed or denied by anyone and to apologize because there are probably 100's of these by now on the escapist. First we to explain where I am coming from we need to start at the Legendary Ending of Halo 3. It left off with Chief and Cortana in a torn apart space ship near somewhere in space. But just before the end of that final scene you see a glimmer and a part of an unknown planet with some rings on it. Now to the part that leads to my speculation, in the movie Halo Legends in the Origins part of the DVD Cortana is talking about the history of the whole Halo universe, then she gets to the part about the Forerunners. When she is first talking about the Forerunners she mentions their home planet and then it shows a planet identical to the one shown on the end of the Legendary ending of Halo 3. This leads me to believe that Halo 4 will take place on the Forerunner planet. The enemies will probably be some unknown Forerunner Technology unleashed by the destruction of all the Halo rings. There will probably be some left over Covenant forces to fight as well. There is also a chance that your new enemies are some kind of species that the Forerunner deemed to dangerous to put back into the universe while they were attempting to purge the universe of the Flood. That's all of my pure opinion, and please feel free to post your theories and speculation below. And again I'm sorry because there are probably 100's of these at this point.