A stainless steel, roughly 686/GP100 sized revolver based on the Medusa revolver concept from one of the major makers but with the Dan Wesson removable barrel system and cylinders and barrels in .45, .44, .40, .38 and .32 to allow the use of .32/.327 caliber, .44/.429, .45/.454, 10mm/.40 and .38/.357 revolver/auto rounds. Ideally, the .32/327 cylinder would hold 7 or 8 shots, the .38/.357 and 10mm/.40 cylinder, 6 shots, and the .44/.429 and .45/.454 cylinders would hold 5 shots.
It would be expensive, of course, but just imagine having a gun that would fire the vast majority of centerfire pistol rounds on the market. You might even be able to fire the bottleneck rounds by mixing and matching barrels and cylinders. The 9mm barrel with the .40/10mm cylinder would allow the use of .357 SIG and 9x25 Dillon, and the .32 caliber barrel with the 9mm cylinder would allow the use of .30Mauser, .30 Luger and .30 Tok.
It will never happen, of course. The first owner would no doubt install the .32 barrel and the .45 cylinder and proceed to blow the gun up.
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An otherwise standard stainless steel Beretta 92FS or 92G with a stainless steel frame. I know that the Billennium and Steel are available, but they aren't the standard 92S or 92G--they are entirely different designs in terms of how the safety is implemented and the frames are plated, not stainless.
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A P-Series Ruger in 10mm. Other than the P95, Ruger has abandoned the P-Series, so this is obviously a pipe-dream.
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Single stack 9mm "carry-sized" gun from Glock. This could happen, but I'm not optimistic.
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A Ruger Super Redhawk with a 5" barrel having a profile similar to the GP100 full underlug barrel instead of the highly unaesthetic "pole stuck into the frame" look of the standard Redhawk. This is a possibility, but maybe there's not enough demand for the idea.