Thinking about accessories like RIS, optics, different stocks? Magazines?

Yeah, but we won't go crazy. I'm not the kind of person who likes to "ridicularize" my guns; on the contrary, I prefer them to be fairly bare-bones. And he's just like me. I can see us maybe buying some type of sight, maybe R.I.S. quad-rail just so that we can have a vertical grip. Even that's kinda unlikely.

Unless you are taking your rifle to combat, a vertical grip is a bit more of a fashion statement. I'd invest in getting a better barrel, trigger and bolt since most AR-15 right from the factory can only do so much for you.

Pressure-bearing parts can improved if you've got the tools, and my Dad's a pretty good gunsmith. He can improve the rifling and twist in the barrel pretty easily. Not sure about the trigger, but we'll see.

I do agree R.I.S. would be a fashion statement, which is why it's unlikely we'll waste the money. But either way, we definitely won't be spending tons of money on sights and other stuff that costs more than the gun itself.

Red dot, EoTech, Aimpoint, or ACOG? So what about flip up iron sights?

Not sure yet. It's not something I'm thinking seriously about just yet.

Well once you have your rifle, you can take all the time.