I just finished Red Dead Revolver and Gun, the only two Old West themed games that I know of and I must say that I believe the genre is worth investing in.  Personally I'd like to see something along the lines of an alternate time-line or TechWest sort of game.  Red Dead did a good job of creating an unreal stylized Old West and Gun did a good job of creating a beautifuly realistic open world in which to ride around in.  The game Darkwatch takes Old West technology but adds a flare of style to make some great looking weapons (not to mention Old West Vampires!)  Imagine a TechWest game with Steampunk style vehicles along with horse and buggy.  Imagine being a gunslinger who is part mechanical and for power-ups you get gears and parts to upgrade your quick-draw speed, etc. 

The duck-and-cover system used in Red Dead is a must but so is the wide-open realistic environment of Gun.   The linear confined levels of Red Dead allowed for some stylistic expression (the quick-draw shootout in the train car) and there's no reason a game can't have both.  You could have chase-style battles taking place out on the plains on horseback and close-quarter duck-and-cover shootouts in the town streets, inside buildings, or inside trains, etc.

I'd like to see a Red/Solid Snake looking character with perhaps a mechanical arm or something along those lines.  For anyone who has seen Vampire Hunter D you know how cool a mechanical horse can be as well.  For the antagonists I'd have an eccentric cast of characters (much like the Metal Gear series) and I'd have to include a vampire in an old west suit, tophat, and cape (which of course must be killed with silver bullets!) And following in the footsteps of Metal Gear and Red Dead there simply must be one character wielding a gattling gun (preferably carried in a coffin.)