Trigger upgrades - I have Jard single stage triggers in most of my ARs and they are a huge improvement. I also have one with a CMC and it's a good trigger also, although I prefer the Jards.
I just put a Timney in a new one and I haven't tried it out yet - while I have high hopes I don't think either it or the CMC are what I want in a SHTF gun.
There is a new Geissele trigger coming out in December that I want to try.
Besides triggers, as has been said aiming devices such as Aimpoint and Eotech.