I've seen the Tau set, and I think I remember a Necron set out there somewhere. No offense to their creators, but they were mostly cut-and-paste jobs... kinda crappy.
I do remember that John Beech of the Universal Shipyards was working on an Eldar set a looooong time ago. The preview renderings looked really good.
View Image Here Too bad he never got around to finishing it.
Emperor's Child, different color variations would be great. Maybe some of us could even create different AI's so they would behave like different Chaos factions.