On the thicker barrels it doesn't really matter so much, but there are some experts who feel that an overtorqued set-screw type can affect accuracy... That's FWIW and YMMV...
I think all the suggestions so far are great. I would suggest that standalone systems would also sell well, like a gas system, BCG as a whole, LPKs, and such-like. Things that can by themselves bring a whole build together.
Adjustable GB can keep you from experiencing overgassing at the user end. May not be a problem at all, but you might stumble upon a particular load that sends lots of nice smelly gas out of your ejection port on every shot.....
Ya know it's really strange, the demand for electricity just never goes away.... <dontknow>
Actually, I've been on strictly 40 hour weeks since the beginning of May...
Follow the links for more info:
http://www.ncwildlife.org/News/NewsArticle/tabid/416/IndexID/9208/Default.aspx
http://www.newsobserver.com/2013/08/02/3070508/gun-suppressors-are-now-legal.html