In any weapons platform there is an envelope performance that gets increasingly difficult to surpass without scrapping the whole platform and going on to another. Hence the reason that bolt action rifles are still the majority of sniper rifles for most of the world. Yes I know the USSR and the SVD blazed the way for semi-auto sniper rifles being adopted across the globe, but seriously count up the number of SASS systems verses manual bolt action sniper rifles and see that the bolt guns beat the self feeders in terms of numbers.
When we talk about the quintessential American rifle, no not the 30-30, no not the Win70, NO not the Rem700, but the AR-15 (come on, stick with me here), then there are some awesome choices to be had. But each choice limits something else in a way that I would like to wax poetic about.
First off is caliber. About the biggest thing you can cram into an AR magwell and still use a magazine is a member of the WSSM family, from 223 WSSM up to 308 WSSM (or 30 Olympic or whatever it is called these days). In places where hunting requires .24 caliber or larger then a 243 WSSM or 30 WSSM makes sense. If you want to shoot long range then one of these "Super Short Magnums" can really ring out the most from the AR-15 platform. At the cost of more expensive ammo, more expensive bolts, and reduce magazine capacity.
My recommendation? Keep your ARs in standard 223 Rem/5.56 NATO caliber if you aren't hunting. If you are going to shoot LONG range (out past 600 yards or so) then you probably want something with more oomph than an AR-15 can deliver at that distance. Yes we can shoot 77gr OTM ammunition out to 800 yards, but even some of the more exotic WSSM or 6.5 Grendel cartridges only gain a tad bit in range on top of that. The AR shines at 600 and under, which is plenty for most shooters. And increasing the caliber (or decreasing it) gets well into the point of diminishing returns except for specialized applications.
Second off is barrel length. You can spend the money to register an SBR with a 7" barrel if you want to, or you can build a "Space Gun" with a 26 inch bull barrel to compete as a "match rifle" in High Power competition (and the guys shooting these monstrosities are posting some damn high scores). Or you can make an M4gery or M16 lookalike to compete as "Service Rifle" if you so desire. Each barrel length gives a trade off between "handiness to carry" and "shootability." Most professionals I've talked to prefer to shoot an M16A2 or A4 but prefer to carry an M4. Trust me, if you go to war you'll carry your rifle a LOT more than you ever shoot it. Trade off here is ballistics, it seems that the "sweet spot" range for AR-15 barrel length is 14.5 to 20 inches. Longer or shorter than that range is into the realm of diminishing returns.
My recommendation, stick with 16 to 20 inches unless you are doing a "specialized" build. The ballistics of a 223 give up too much below 14.5 and don't gain enough beyond 20 to really justify those lengths. Don't get me wrong on the lengths beyond 20 inches, the 223 does gain some velocity, but you are well into the "realm of diminishing returns" at that point. This goes back to caliber, a 223 WSSM out of a 7 inch barrel is kinda stupid (but do what you want) while a 9x19 out of a 28 inch barrel is equally as stupid (once again, do what you want).
Third is weight. You can make a 5 pound ultralight or an 18 pound monster varminting rig. If forged aluminum is too heavy you can get a polymer lower. If you don't like thick barrel you can get a pencil barrel. If you don't like recoil you can add weight to the buttstock.
My recommendation? Unless you are doing a "specialized" build (such as a lightweight backpacking rifle, or predator calling rifle, or ultra heavy prairie dog gun) stick between 6 and 10 pounds total weight with any accessories you feel necessary. You can go crazy making an ultralight gun that opens up groups after three shots or go crazy making that prairie dog sniping system that won't change POI until you've shot a hundred rounds. Your choice as to what fits your needs, no wrong answer here. Although at some point if you are looking to reduce weight it is just cheaper to work out more often. Getting below 6 pounds can be costly, and getting above 10 pounds is usually costly, so unless you are making a specialized rifle you are into the point of diminishing returns.
Fourth is accessorizing. You can put rails all over the dang thing if you so desire. Yes I've made use of a quad rail system to mount a flashlight, PEQ-15 and grip pod when I deployed. However you can go with old school handguards as well (and we fought plenty of wars with plastic handguards).
My recommendation, skip the quad rail unless you actually plan to put something on there. You don't need a Brigade Quartermaster's worth of accessories to shoot well. If all you want is a place to attach a bipod you have other options. But it will always be your rifle, so do what you want. Sometimes looking cool matters in the gun world.
Lastly, recognize that the AR-15 has some pretty significant limitations. It is not an ideal platform for 1000 yard shooting (you can do it if you want), nor is it an ideal platform for shooting one handed while hanging from the skid of a helicopter (you can do this too if you want). What it is pretty ideal for is making a rifle that does what you want it to do within the limits of the platform. Good shooting.