I'm a 9mm guy, but I always got a kick out of longer-range handgun shooting. So, that said, I might say an accurized 1911. Maybe something that will shoot at least 8MOA (at 100 yards) would be sweet.
Then again, if I am designing it, I'd probably go ahead and use a similar design that allowed me to tune the muzzle-end lockup, but as a double-stack 9mm. Or, maybe I'd go with 10mm instead, now that I think of it...