At the end of the Halo 3 Legendary ending, the half of the Dawn carrying the Master Chief is seen heading towards a stange planet. What planet do you think this is? Evidence would be nice. In my opinion, it's the Forerunner's anicient homeworld, because of the geometric lines and Forerunner glyphs seen everywhere on the planet's surface from space. These glyphs are also seen on the Halo: Legends episode "Origins 1" on the Forerunner homeworld's surface.
Yeah at first most speculation was onyx, but after that halo legends vid i would go with the Forerunner homeworld, which in fact is the Precursors homeworld that the forerunner found and adopted (basically who they learned everything from)