Well since you asked
1. Adjustable Gas Blocks. I have a SLR rifle works adjustable gas block on one of my ARs, it's nice, works well and with the JP Silent Capture Spring system it can be tuned to your ammo. No over gassing and less felt recoil. I can also shut off the gas system so I do not have to chase brass (or use a brass catcher) when shooting off the bench.
2. JP Enterprises Silent Capture Spring System. You can get rid of the buffer retainer screw on your design, making it easier to clean and you don't get the 'twang' in your ear while shooting.
3. Different receiver end plates, some people like QD's, some like snap hooks, lots of different types available. Maybe have people supply their own if they want something unusual?
4. Nickel Boron Bolt Carrier group!!! Great Idea, I have a Nickel Boron BCG made by WMD in one of my AR's, easier to clean than say the 'Mil Spec' one in my Colt Canada SA20. +1 for the Lantac NiB BCG, they seem very nice, were not available in Canada when I was building my AR or I would have bought it. Most people don't have the luxury of a ultrasonic cleaner at their house, so easier to clean is always good.
Maybe even offer to modify customers bolt carriers to use in the rifle, so much selection in bolts & bolt carriers out their now. Some want NiB, other lightweight, etc.
5. Different Safety Options. Maybe a Short Throw Safety?
6. Lantac Dragon Muzzle brake. I love the one I have.
7. Maybe you could sell them like Colt USA does with their 6920 series. Specifically their 6920-OEM2 rifle, that comes without handguard or buttstock or magazine. basically stripped. See link below:
http://www.colt.com/Catalog/Rifles/LE6920Series.aspx
I know I have a rather large selection of AR 'stuff' at my house & would just try different buttstocks & handguards on the rifle until I found ones that 'felt right'. Then I would order whatever I wanted if it ended up being a duplicate of a buttstock/handguard off of another rifle. Or for people on a budget they could pull some stuff out of their parts bin (or cannibalize their existing rifles) to throw on for now.
8. As others have said, caliber options. I know you are opposed to this for reasons that have been stated a few times in other threads. But seriously quite a few people will want to hunt deer with this rifle. My vote would be for 6.5 Grendal or 6.8SPC. You already sell 6.5 Grendal in your store!
9. Option of a 2nd upper. Either stripped so we can build our own upper (I know you have said that this is a no go, but people are going to keep asking), or complete so we can cover the heavy & long vs light & short barrel options.
Too bad the upper is not the serial numbered part like on Swiss Arms rifles. If that were the case I could realistically get rid of half my ARs and replace them with one MV lower and several uppers.
PS. Now for my attempt at humor.
I personally don't want a TiN Gold Coloured BCG, looks a little too bling for me as other have said. But if you do go that route, you might appeal to a different type of buyer and could potentially expand your product line. You could also offer other accessories like:
a nice high crown flat brimmed ATRS ball cap, with sticker on brim
diamond studded ATRS phone cases
ATRS bandanas
could make a sweet collector set of firearms if you sell with a TiN Plated 1911 or Desert Eagle!
