What nobody has mentioned...so far
What you need for your collection is a Single Action Revolver!!!
Yoy've gopt a lot covered already, but I dodn't see one of those on your list. Historical significance, you bet. If you want straight history, get a Colt, or a clone. If you want a great gun, get a Ruger. The VAQUERO for traditional looks, or the Blackhawk for the most functional shooting machine.
Any caliber is ok, but bigger bores are more traditional. .45 Colt is a fine round, and IIRC you can shoot them in your .454 if you want a plinker round for it.
.357 (with a 9mm cylinder) is a good choice, or a .45 Colt with the .45 ACP cylinder is another. .41 & .44 Mag, or if you really want something oddball, .30 CARBINE. Or you could go small bore, and get a Single Six in .22LR/.22Mag, or even one in .32 H&R Mag. Ruger makes the best "modern" single actions on the planet in their price range (Freedom Arms are very very good, but more spendy). Coil mainsprings, transfer bars (so you can safely carry six loaded), ok triggers that can be tuned to very good, and Blackhawk models have adjustable sights. I have owned several Ruger SAs for over 20 years, and put a lot of rounds through them, and I am currently in the market for a couple more.
You'v got defensive autos covered, and the big DA monster mag, why not add something that is just fun to shoot and use, along with the fact that they do still have practical uses as well.
Or, as another alternative direction for your collection, go with the single shot. T/C Contender! I have nearly a dozen barrels for mine, from .22LR to .45-70. I even have a 9mm barrel! You don't know how well you shoot a handgun until you start shooting the single shot. It is a whole 'nother level.
Here are a couple of ideas, just for fun. Whatever you decide to go with, enjoy, but consider what you already have, do you want more of the same, or something new? I do agree with others that you ought to have a 1911, and a S&W M29, but for your next toy, why not think "outside the box"?
