By the power of truth, I, while living, have conquered the universe.
Why the hell would it be Reach, Harvest or Sanghelios? There's no logic as to why it would be any of them.
It's clearly a Forerunner planet, the Shield & Sword glyph that has been on every Forerunner installation is pretty visible on the surface of the planet.
Given the nature of the final Terminal, Mendicant Bias pretty much tells us he's sending John to the place the Forerunners exiled themselves to in order to prove to them that he's atoned for betraying them during the Forerunner-Flood war 100,000 years before.
[Edited on 12.10.2010 5:16 PM PST]