Many years ago, I started with one of these:
To that I added and discarded various gear over the years, eventually reaching this stage:
Sierra Precision Grip - filled my hand better and kept trigger finger at the correct distance
PRI Gasbuster - easier manipulation, better charging handle, and future suppressor expansion
Upgraded the old school CAR stock to a newer enhanced M4 stock
Replaced the single-heatshield handguards with M4 double-shield handguards. The gun would just get too hot to hold in the summer.
Replaced A2 upper with flattop upper.
ARMS #40 sights to use as back-up (cutting edge at the time)
For primary optics, I went with the TA11 ACOG on a then brand new ARMS #19S mount. I had been playing with ACOGs for awhile and was very pleased to have my own personal ACOG.
A Bushmaster/JPS Patrol sling - two point but the end would slide off for transitions. Still a pretty good sling in a lot of ways.
That was such a good rig for me, that there wasn't a lot of room to "improve" on it; but there was SOME room.
I added a Magpul M93 stock. I had initially been skeptical during the pre-production trials of the stock; but after hearing good press I decided to give it a try. One of the better AR purchasing decisions I've ever made. It balanced the gun much better, was solid and a definite improvement over the old stock.
I also added a KAC FF-RAS (7.0 of course), a QD KAC sling and a CQB (Now Specter Gear) SOP 3-point sling. The RAS gave me a lot more flexibility and a smidgen more accuracy off the bench by free floating the barrel. However, the nylon collar of the JPS sling and the aluminium RAS did not get along well with heat, so the new SOP with transition buckle was in order.
Now that I had rails, I needed something to hang off them; but I liked using the magwell instead of a vertical grip and a bipod was more weight than I liked with the HBAR profile. I threw on a handy Surefire G2 Nitrolon in a low-mount 1" Millet ring until I could get a "proper" weaponlight. Unfortunately, this worked so well I started to lose the urge for a dedicated weaponlight at 10x the cost.
I also replaced the SOP sling. Three-point slings and I just weren't getting along. It was always tangled on something. I missed being able to use the sling as a support; but I needed something with fewer frills. I went with the CQB (now Specter Gear) MOUT sling. In addition to being very easy to use, it also allowed to to clip in or out of various weapons quickly which I found very convenient for recreational shooting.
I was pretty pleased with the rifle at this point; but a friend had let me shoot his Armalite midlength and the easy, smooth recoil of that rifle (and better sight radius) was haunting me. The old HBAR barrel had almost 9k rounds on it and wasn't performing quite like it used to... so I decided to rebarrel and lo and behold, I received a midlength barrel by mistake.
I took it as a sign from above and sold the KAC FF-RAS to get money for a DD 9.0 rail. I also decided to try the PRI folding front sight because I liked the way the hood and post gave a very fast sight picture up close (post within circle within circle).
Finally, I added a Magpul trigger guard that came in a package with my pre-production MIAD grip. The MIAD was nice; but I still liked the Sierra better so it went on another rifle. The Magpul trigger guard on the other hand went straight on to my favorite.
You would think this would be enough for just about any sane man; but I had long passed the point of sanity. There was still room for improvement in my mind. I replaced the long-serving and perfectly working ARMS #19S with a Larue mount. While the ARMS gear was doing the job just fine, I have to admit the Larue stuff is better. I also replaced the ARMS #40 with a Troy. Once again, the ARMS was doing the job; but the Troy did it better.
In my quest to build a rifle that could do a box drill in under 5 seconds and then hit targets with a degree of precision at 600yds, I added a Grippod to the rifle. The Grippod let me get better control at close range and doubled as a mediocre bipod at long range - and at only 7oz weight.
That's the current evolution of the original AR; but probably not the final one. Even now optics that make better advantage of the precision capability and a suppresor are looming in the future...