In away I see the Forerunner like a race in a old Star Trek Next Gen episode. In the Star Trek episode the Humans, Klingons, Romulans, and I forget who else, discover an ancient artifact that they assume is a super powerful weapon. They without trust work together to uncover the artifacts mystery and eventually come to a key planet. There they discover that the artifact isnt a weapon but a piece of Ancient(in the cosmic scale) History. A hologram of sorts appears and tells the assorted species the tale of its race; how they had lived so long as the only sentient species in the galaxy that they eventually started to die, as though the species had become genetically stagnant. They then created all the sentient races in the galaxy before they disappeared forever.