Mendicant Bias
Forerunners survived,Precursors survived, Flood (although weakened) survived, though debased Man has survived. 

It seems as if the universe has a ennui towards Humanity with them being able to survive whatever is thrown at them yet being withered away considerably with each event. While enduring the Covenant war and looking moderately strong post-war in no way could Humanity survive another engagement of that magnitude for a century or two to come. Though seemingly introduced and forgotten a collective of Human servitors known as The Assembly tasked with the oversight of Humanity,protecting them from hasty technological advancements and ....Forerunners.

Considering The Assembly was able to survive somewhere within Sol under the radar of Didact, Librarian, and the Ecumene itself is it not possible for another AI group to be out there if not Humans themselves?  As any civilization Ancient Man no doubt had backwater or black worlds which were unknown to few if anyone at all which could house  AIs if they aren't hidden infront of us once more. Ancient Humans themselves could survive had they considered some form of sublimation akin to Composition and transferred themselves within robotic shells or  stored being of importance within Timebolts during the Forerunner war. To this day there are Human planets who so far flung the Covenant war is nothing more than a myth , how unrealistic would it be for an iilos among Ancient Man surviving the passage of time?

I ponder this because Forerunner influence became a significant plot point during Halo 4's era with it essentially seeming all of Man's advancements being tied to a Geas,imprint, or general Forerunner intervention. What of us is Human if every major advancement doesn't stem from our own ingenuity and struggle? Ancient Humans having technologies purposely created or by chance aiding in the development of modern Man would be a excellent counter balance to the growing dynamic of Forerunner intervention being reason for Man's supremacy.
Also having a pocket of Ancient Humanity survive by some arcane means would also add more hubris to the Forerunners themselves.
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Venture Jones
I only hope there is some use of Ancient Humanity in Halo Infinite, with the ring that is in place for the game (supposedly).
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Zo Resken
The idea of Ancient Humanity playing a role in in the present era of Halo via the Assembly is a very interesting. There is no reason not to believe it is possible for Forerunners at the end of the Forerunner Human War missed a set of Ancient Humans who turned themselves into AI. The only way I could see actual Ancient Humans return is if they came back from a different galaxy due to the firing of the Halos. The Created Storyline is a great opportunity for the Assembly to reappear and hints if read in a certian way could point to Ancient Human history on Installation 07 playing a role in Infinite. 
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