I'd like to see a piston driven upper for the AR with the hottest loads that can be packed into the small size.
A 50round box of 5.7 is tiny compared to most ammo out there. If full .223 velocities could be achieved out of a small round, that would have huge potential. I haven’t looked into powder specifics, but I’m sure there’s something out there that could make a 5.7 +P++. Perhaps make the round slightly longer so that it can’t chamber properly in a 5.7 gun and also make it too tight of a fit in the current magazines that would alert the user that, hey, it’s the wrong ammo. Now if it was backwards compatible with current 5.7 ammo, that would be sweet. Possibly a lever to change the amount of gas escaping similar to that of the FS2000 to ensure reliability.