San'Shyuum Reformists
Remaining factions of Covenant / Covenant unaware that the war has ended
Prehistoric Humanity
Surviving Forerunners
Forerunner Tech (Sentinels, Monitors, etc.)
Surviving Precursors
Flood
Edit: We can't fight the last precursor... He's 100% dead since the end of H3 when we blew up Installation 04B right? He definitely died... he was on the new Halo and we blew it up. I've said previously that a Gravemind's consciousness might outlast their physical body - as long as there is Flood to command, the Gravemind might persist in a kind of out-of-body state. That way the Gravemind of 2552 might be the same Gravemind talked about in the Halo 3 terminals during the Forerunner-Flood war, and that way the last Precursor might survive past the end of Halo 3...
[Edited on 07.03.2011 2:13 AM PDT]