The things I usually add on or change on my AR's is:
1. The stock
2. The grip
Its hard to fall in love with shooting a rifle if you arent comfortable shooting it. I usually try "upgrade" the components that play most in the ergonomics and shooting experience.
My first AR build was a 300blk. Loved the gun and the round. only issue I had was ammo availability. Ammo can be found just have to always be on the lookout, that is unless you reload.