Brilliant idea, This would be enough for me to get in line for a modern sporter, or even just buy for future builds.
I think something pretty general would keep most people interested until purchase.
Faxon gunner type profile
Black
5.56/.223 caliber 1:8 twist
Mid or Rifle length gas port
18.6 total length (16.6" barrel, 2" muzzle device)
Flash hider over muzzle brake for not having side concussion
No open fork type flash hider to avoid "ringing" also to avoid snagging
Something with similar design, but better aesthetics than an A2, (like a Surefire socom closed tine flash hider)
This would likely be a safe setup. I'd preorder in a heartbeat