First off, MAG Tactical advised that they have a magnesium lower receiver in the works for early next year.
Next, I'd pair that up with a slab side DPMS upper. To get extra light I'd suggest the JP LMOS BCG. Although a full size BCG isn't going to break anyone's back.
Phase 3 would be a Fulton Armory 16" lightweight barrel with a low profile gas block (adjustable if you have the LMOS). With a BABC on the end... of course!
Personally I'd wrap the barrel in either the new Midwest Ind Gen2 SS tube or an APEX "Gator" tube (sans top rail). All depends on budget.
Furniture from there should be user friendly. I think the Magpul MOE+ is awesome, and an ACS-L to match would be nice.
A single stage trigger, no sights, fixed 4 scope, heavy buffer, short throw BAD-ASS and a padded VTAC sling would finish it off for me.