Here's an idea. Maybe the dev team, once SotP is finished of course, could look at a possible expansion involving the Alliance between Ancient Humanity and the San Shyuum, vs. the Forerunner Eucemene, vs. the Flood? They could call it "War for the Mantle" and have it set 150,000 years ago during the Human/Flood/Forerunner wars.
I expect SoaSE to be old news by then.