First of all, when you say AR, do you mean Armalite or Assault
Rifle?
I've always be very amazed at how well-loved the AR-15 platform is
by Americans. (Of course, I love it, too. I have handled some other
weapons like Steyr and MP-5 but the ergonomics aren't as good.) You
guys have made everything from AR-15-based pistols to AR-15 based
belt-fed GPMGs. Mostly useless and more in the category of
because-I-can projects. 
For civilian use, the original M-16 gas system is probably good
enough(?). Sure, it starts clogging up after a few dozen live
rounds but a well-maintained civilian rifle should still be able to
keep shooting with little danger of IA.
Unless you're with some militia training seven days in the bush in
preparation for armageddon, I don't think you'd be looking at
firing a 100 rounds a day. Would you?
For military use, the piston option will be fantastic. Taiwan has
already out into service (and also exported to Jordan) their own
version of a piston-driven M-16 the T-91. Looks really neat.
Since Americans love the AR-15 system so much, I really can't
understand why the US army continues to waste time and money to
R&D other systems like caseless, XM-8, etc. The answer is
obvious: the next US Army weapon should still be a AR-15-based
weapon, probably piston-driven. US Army don't like bullpups anyway
and I feel that as far as conventional assault rifles go, the AR-15
can't be beat.