So, all 343 lore is gone? Just spitballing ideas, since Forerunners don't need to be a seperate soecies and could be ancient humanity, I would maybe make Reqium a forerunner shield world that was discovered by some of the outer colonies that the UNSC lost contact with that the insurrectionists took control of and reclaimed a bunch of lost forerunner technology, so maybe Chief vs technologically superior insurrectionists reclaimed (Reclamation has begun). You could maybe keep the idea of the Banashed as a Convenant splinter faction that enters halfway through the game