Before making any recommendations, I would ask what you are building it for? For instance, it looks like a mid-range target shooter, maybe longer, and I think you have it fairly nicely set up.
If this is for in-home or in yard self defense, I would consider dropping the scope and add a rifle red dot with a matching three power scope that could be folded over out of the way when the scope isn't needed. Vortex and Holosun make some nice combo red dot and matching magnifiers or scopes. And add a light. Because of size, I really like the inforce light. Small, not a flamethrower, and wide beam with a good hotspot. And if you want something else, I'm a big fan of KISS - no strobe, and no multiple brightness settings. On or off, and momentary on are all you need. Complexity adds training requirements as well as a major opportunity to get it completely wrong and maybe then lose focus on the threat at the worst time.
For SHTF, a mil-spec 16" barrel, your scope, and a sling so that you can access a pistol, water, food or take a break. Really look hard at an M4 from the sandbox for goodies.
And for a long distance shooter, a really good scope, a really light trigger a bipod and a 20" barrel.
Because I imagine various scenarios, on both of my pistols and both of my carbines I have added ambi charging handles, ambi safeties, and with Magpul's BAD lever, ambi bolt releases. The long distance rifle doesn't need all of that, but add what you wish depending on how you envision using the rifle.
Basic advice for anybody thinking about a build is to thing really hard about how you intend to use the pistol or rifle, research the parts you think you will need/like and can afford, then make sure all is compatible and build away.
Another thought kinda discussed in a bunch of earlier replies, and that is mil-spec is good enough for the military, so that is all you need. Yes, I agree, but I don't think most of us are going to put the rounds downrange like a sandbox M4 will. So, while I recommend good quality parts, going off the mil-spec reservation is ok as long as the new part enhances the function of the pistol or rifle in regards to the purpose of the gun.
Last thought and I'm done. Try to avoid the latest and greatest gimmick unless you can really convince yourself that it enhances the purpose of the gun. Avoid complexity, and really ask yourself if the new part might increase your chances of fumbling the mission. For example, a 2lb trigger might not be best for your in home/yard self defense gun when you are experiencing tunnel vision and compromised fine motor skills due to adrenaline and whatever else.
And train exactly for your mission. Good luck!
