I think ND would be able to put a very clever twist on that idea. They could have the Precursors, Dark Makers, and the Metalheads explained all in one game. The reason why I think it would be necessary to include the Metalheads into the plot is from Jak 3.
"The Dark Makers don't play nicely with others, just ask the Metalheads."
Obviously, the Dark Makers must have done something awful to the Metalheads in the past.