I'd love to see a dedicated 9mm AR lower that takes Colt Mags that doesn't look like it was made for 5.56 mags: i.e. the QC-10 9mm lower: the magwell is slim so it looks like it goes in line with the mag.
I know the Glock mag lowers are all the rage as they hold 10 rounds, but dammit, the straight Colt mag look is dead sexy... '80's SMG style.