The following link details a GDC 2013 panel from yesterday 3/27/13, with Scott Warner, Halo 4 project lead designer.
In the article, Scott describes the early on development of the Prometheans. Most notably that there were SIX Prometheans initially before 343 decided to cut that in half to the three we got in the game. Though he goes on to say that they hope to get the other three in the series at some point. Hmm, DLC??? Spartan Ops Season TWO???
The reasoning behind the Prometheans was to give long time Halo fans new enemies to fight as they (343) were concerned about player “fatigue” with battling old enemies.
The first Promethean to be created was the Knight, followed by the Crawlers, then the Watcher.
The article is an interesting read (but for the typos).
-Sal