A 20 gauge pistol would be a Sawed Off Shotgun. The Taurus Judge is a .45 Colt pistol that can shoot .410. It is a pistol. You can't make a 50 caliber plus pistol, as it is a Destructive Device, and you can't make a shotgun pistol, as it would be a sawed off. Thus, no 20 gauge pistol can be made under current laws for widespread commercial sale.
One of mine would be a .357 autoloader that wasn't a Desert Eagle. Fortunately, Coonan is back in business. The only thing more badass than a 1911 is one that shoots .357.
I would like to see a shotgun that has both an internal tube mag and a box mag. That'd be hard to do.
I'd like to see a new 22lr pump action gallery gun like the old Winchester.
I'd like to see an M1A in 30-06, modern 30-06.
A double barreled tube fed semi auto semi auto 12 gauge would be interesting.