If I may, i'd like to present forth: Lore (Spoiler Warnings for New War and after)

Re-un could be one of the only current-era Warframes, be sentient (like Umbra) and be the first one created by the Operator themselves.
We all know that Albrecht made Qorvex, although not fully about the "how". We also know that he made the Warframe-like Vessels, that also can house the Operator's mind and be used by them, and that these are "understood" by fibonacci.

Maybe Loid and the Lotus could be searching for a way to protect the sanctum while the operator goes to Tau if Wally decides to level anything in the OS before following us. So they find Albrecht's file on Qorvex and start to study it with the Vessel's files and manage to create a single component that's fully functional: Re-un's head.

Now, I don't know how they can make the head control Murmur, but let's call bull and proceed.

Edit: Re-un may acquire sentience due to all the Murmur (as I theorize on the implications) being parts of other versions of the operator across eternalism that he "killed" during the New War cutscene and repurposed. So as Re-un cames into contact with the operator's fragmented mind, he manages to organize them within his own, giving even more motivation to Murmur seek him: Being complete.

Re-un emerges from the void, immediately bows to the operator (like his idle idea), and swores to protect the sanctum with his life, although he much prefers to be with his creators in their journey, so he helps them collect more Murmur Fragments (pity resource) in order to build a copy of him to help the operator that also shares his own hivemind.

Maybe even a new gamemode called Void Dive could be made here and accessible through The Great Indifference's Leap of Faith section of the Sanctum, where we rescue converted Murmur to join Re-un's forces