To me the key is a firearm family that the lower part is the firearm, like the AR or MAC style RR or you have to make the parts fit the receiver of guns like an Uzi. It would also seem to me that it would be much easier to use or modify "off the shelf" parts than designing all new parts. Think of how difficult building an MP5 style roller lock, if HK had never built the MP5. If 53 parts needed to be adapted for pistol caliber use. It could be done, but it would not be easy. But those part are on the shelf.
I think most would agree that the MP5 is the top of submachinegun development. Submachineguns have been replaced by very short barreled medium caliber firearms. Here in the US there are no "new" designs available as transferables ended in 1986. Why are HK sears and trigger frames at the top of transferable rifle, carbine, or subgun? They work well and can be all of those things plus it has multiple caliber belt fed options. The reason why I brought up the M16 and MAC style RRs is I think both could be converted to roller locked pistol caliber function the easiest as the "upper" would have no restrictions of shape. The pistol caliber roller locked mechanism and I think would be a step up for both of those firearm platforms. HK sears are great. I feel fortunate that I own one. But with a $30,000+ entry fee, I'd think adapting the mechanism that makes a HK sear fire would improve those other platforms at least in the pistol caliber. YMMV.