Would be great to have some ship-mounted high-rof low caliber weapons. Recent events showed that ships need some better anti-infantry weaponry.
So I'm all for something like M134 (7.62mm) or GAU-19 (12.7mm, with sexy tri-barrel). They'd be much smaller than current weapons, so could use smaller turret/pintle mount for more precise control.
M61 (20mm) using standard weapon body would work too, against unarmoured vehicles. Would give a reason to use single layer of plating.
GAU-8 (30mm) would basically act as 10 AC glued together, as it would most likely use the same ammunition. So it doesn't bring much to the game (other than visuals).