Personally, I think an MR556 10.4in pistol would sell every example they make. Double points if offered in .300BO and tuned for suppressed/unsuppressed. Same for a UMP-45 pistol ala SP5K. Not that I particularly love the UMP, but HK collectors would ensure it stays backordered forever.
A 10mm version of the HK45/HK45C with an RDS plate would sell extremely well, I'd buy at least two of each. I have a USP40 I bought for a 10mm conversion project that was made possible by a run of 10mm barrels made by Michael's Machines. That part of it is perfect (though changing the recoil spring is a PITA) but the magazines are really the problem.
Fun fact - an HK45 10rnd magazine holds 16rnds of 10mm. It's my grand handgun folly. Currently, I'm patiently waiting for L&M Precision to start accepting USP RMR milling work.