3 pack extractor kit w/extractor, spring and insert (or at least the 3 pack spring and insert kit and 1 extractor), and a couple packs of gas rings wouldn't hurt.
Nor would an ejector and spring, but might be beyond 'general need' if not a volume shooter and/or don't have proper punches to do the change.
Charging handle never hurts.
If you can swing it, a whole BCG would be of use in the event of a bolt (previously mentioned) cam pin, firing pin, firin pin retaining pin or gas key failure/damage/loss
I kinda like to have at least 1 LPK on hand as a spares source on top of the plano box full of bits and bobs.
(front sight posts w/detent and spring, rear sight parts, stuff like that)
But then again my range bag also has every carbine buffer weight from standard up through H3 LOL