I love conversations like this because SMG's are very cool even if they're considered a bit outdated. One role I can see the SMG shining in in the modern era is their ability to dominate in a "pistol" fight. Within reason, a lot of SMG layouts are almost as concealable as pistols. I've only ever shot two SMG's and those were a stamped M-11 and a stamped Model A Uzi. The shitty wire stock and high ROF turned me off to the M-11. The Uzi was surprisingly controllable and the underfolder was a lot better than I expected. It's a fucking chunk though and you can kind of tell it was done in a hurry and meant for soldiers and not spoopy shit. Maybe a more modern, closed bolt design that incorporates a rate reducing pawl like the AK uses would be good? I don't know a ton about the various, uncommon loads but how about just some hot 9mm AP? Maybe if it had a similar design to Russian 7N1 with an air pocket near the front and a steel knocker? Again, I'm not sure how well that design would work in a pistol round since it's originally like 152gr and fired from a rifle. There's just something that turns me off about using uncommon rounds but I guess if the round was good enough, it wouldn't be uncommon for long.