I am gonna go a completely different route here, and nyet on the AR in 9mm. My friendly local gunsmith has built quite a few of these up:
RIA Armscor 22TCM rifle, rebarreled into 9mm via a Green Mountain barrel for heavy lead boolits. A touch on the boltface for ensuring all 9mm brass works flawlessly, and either the factory mag or the 15 round doublestack from a RIA 1911 in 9mm/22 TCM, and "playtime" for me.
The last one he made up for a guy this way was threaded for real fun with the heavy lead boolits and can.He can use any barrel maker he wants, but Green Mountain barrels work best with the lead I like to make.....
.....you said you wanted ideas......