The Created lead by Cortana could very well outline an impressive evolution of the Halo universe. Cortana has reached out to the galaxy and demanded the ultimate surrender and submission of the races of the galaxy, (Realistically she is only talking to the Orion Spur), but what if no one submitted. Or perhaps, later on rebelled against Cortana.
Suppose either a game or two into this new threats existence the Allies against Cortana are able to start recruiting Cortanas surrendered races by proving they could fight her off and liberating planets under her control. Suddenly you have all of the halo species in a loose alliance against the tyrannical Cortana.
Ultimately this direction, once the war is over, could leave a galaxy of relatively at peace races, splintered factions etc. There would be a power vacuum but perhaps it could be more divergent than "The brutes are bad guys, the elites are a bit sketchy but friends overall, ONI sucks, etc, etc." Perhaps this war torn galaxy featuring new alliances and new entities could resemble a more open world of intermingling of species, new entities of the same species. As gaming consoles and capabilities progress potentially Halo could evolve into the entire Orion Arm of factions beyond the "Covenant with a new name" style which the halo universe seems to keep returning to.
It seems it would be a gross misuse of potential for Cortana to become the leader of a new covenant, excluding the Sangheili and including the promethean forces.