I'm not a Halo scholar but I thought of what I personally believe to be an interesting idea on how the Forerunners could return to the universe in an alive, but diminished form.
So I know the dev team are waiting until they get the Covenant/UNSC nice a polished before tackling the potential Forerunner remnant faction. It seems to me the devs don't know exactly how they want the Forerunners to return(IIRC they are leaning towards shield worlds and stuff)....so I'd like to suggest the following:
100,000+ years ago the Librarian went on an expedition to the dwarf galaxy Path Kethona, or Large Magellanic Cloud as we know it to search for the origins of the flood. She discovered a huge, ancient Forerunner fleet there and a single planet inhabited by an ancient and prehistoric Forerunner society as well as many Precursor artifacts strewn throughout the galaxy.
On this planet, there was a biological knowledge depository detailing the Forerunners rebellion and eradication of the Precursors 10,000,000 years ago. It would also logically contain info of their original home, the Milky Way. So as this society matured, and reached out into space....they would encounter the same Forerunner and Precursor technologies that the Librarian did, they could have used this the greatly accelerate their technological growth to levels we know the Forerunners for. At the same time, they would learn more of their lost home, the Milky Way....and, for the sake of our story, yearn to reclaim it.
They would learn much of their race and civilization, but they would be ignorant to the current state of things.
http://www.halopedia...to_Path_Kethona
