Something that allows you to swap out the stock for either a collapsible or fixed stock. removable carry handle with a picatinny rail underneath. fixed carry handles are dumb. removable front post is also a bonus. quad rail hand guard. 16" barrel
with that very basic setup you can configure your rifle (with additional parts of course) to be a long range sniper rifle (fixed stock, high power scope with picatinny rings, and a harris bipod that attaches to the quad rail fore grip) to a CQB rifle (collapsible stock, reflex sight mounted to the picatinny rail, and a verticle foregrip)
also, this might be harder, but the end of the barrel is threaded and you can swap out muzzle brakes or flash hiders