A MAC 10. Hear me out.
Masterpiece still makes them but they changed them into something off brand with no compatibility. A LAGE outfitted MAC is a very good gun. If MPA can make them so cheap in the U.S., you could make a modernized version that would sweep the sub market.
Beyond that, a simple plastic shell and new grip panel could turn one into an MP7 clone. Not like PSA’s, but one that actually looks like it and takes a better caliber.