I'm not saying they would create their own domain for technological reasons or even need it for any other purpose than to infect any surviving forerunners with the logic plague along with all others AI's. The original GM saw how well it was working to have one (extremely powerful) AI against the Forerunners. Even being able to shut down separate forerunner armor systems. After the original GM learned of the domain he decided to create his own as a trojan horse, or fall back idea. After the latest GM torture and interrogation of Cortana, couldn't it be possible that he learned just how much of human society was ran from AIs and is using his new "Domain" to spread his logic plague on an intergalactic level without having to have direct contact with any of them, therefore the remaining sentient societies would never see the flood coming because they're disoriented from Cortanas attack.
I've been running with the theory that it is a flood domain. I assume at least some data about the domain was stored before the fall of the forerunner, if so naturally MB would have told the GM about it before their fall. Could it be possible that the original GM, with the knowledge of the precursors, was able to create their own "domain" as a fall back plan. It seems that the GM's have plenty of time on their hands so maybe it was a long term fallback plan MB and GM used as an contingency for if the Halos were successfully used? It fits in nicely with the Cortana logic plague theory. All conjecture but I like my theory. HahaWhy would the flood need a domain? They have neural abilities so powerful they can literally think thoughts across galaxy's. And given how much data a single human brain is capable of storing a gravemind has the combined processing power of millions of humans. It's literally like a human alien super computer so why make something like a repository when your literal existence is a repository.
I think we are giving the Gravemind way too much credit here. As powerful and intelligent as it was it was driven by a single desire, consume. Correct me if I'm wrong but I do not recall any evidence of the Flood building technology. Not to mention at the time Humanity was just a insignificant piece of meat to it as we were on the brink of extinction and had no weapons that could pose much of a threat. There was one and only one goal of the GM stop the Halo from firing so it can consume all life. And it accomplished it's goal but could not account for how crazy John-117 is.
To me personally, what you are suggesting is a backup plan of sorts. Had John and Cortana the person you are suggesting is allied with it now stopped it on the Ark it would have been game over for the entire Universe.