These are some vague ideas I'd like to think about publicly for a while. D&D is, of course, an adventure game, aimed at a particular type of adventure: braving monster- and trap-infested areas and returning with wealth, with the eventual goal of clearing wilderness and establishing a stronghold. Very early on, other designers tried to repeat the D&D success in other settings; we saw space games, westerns, superheroes, and horror games. Some concepts were more successful, others not so much.
It's been suggested that the failures are due to trying to fit the new settings into the D&D formula. Take westerns: trying to play D&D in the wild west doesn't really fit, because even though you could substitue mines for dungeons, the typical western doesn't really look like a Fafhrd and the Grey Mouser picaresque adventure. It's a different type of adventure. So, typically, western adventure RPGs get used for one-shots based on cliché western scenarios: the posse, the shoot-out, the indian attack. People seem to have a lot of problem with how to run a western campaign, or campaigns in other settings which, like westerns, don't fit the dungeon-crawl formula.
What I've been mulling over is how to identify the elements in any given type of adventure that could be used for a simmy, sandbox or sketchbox-style campaign. In other words, how do you turn a western concept into a sustainable RPG instead of a tool for one-shots? I'm actually interested in doing this for other settings, because I'm not a big western fan, but I thought I'd start with westerns because I think I see a couple clues.
The problem with "D&D, but in the wild west" is that standard D&D fantasy is basically an endless caper, while the western is not primarily oriented towards the caper. Sure, there are western capers, but there's no endless source of wealth, and most western capers look very much alike, which is why it doesn't make for sustainable campaigns. What the western focuses on much more is the fight against lawlessness: it's sort of like the D&D endgame, but with an unprotected town instead of a stronghold, and this is where you start, not end; the scenarios that most western RPGs focus on, like the shoot-out and the posse, are situations generated by the tension between lawlessness and the urge to civilize, not ends in themselves.
So, I think the western campaign should look something like Deadwood. You start with a barely-settled area and a potential source of town income (mining activity) and plenty of sources of violence; the PCs attempt to build the camp into an honest town, make improvements, fight off the lawlessness. Random events will occasionally stir up new trouble in the already volatile mix.
A space frontier campaign (something I'm more interested in) would be similar: establish some bases and trade routes and threats to both, plus chances for various random catastrophes. The on-going campaign is about building up resources to cut transportation costs for necessities, establish defenses, wipe out pirate bases, deal with rebellions and feuds.
And an atomic horror campaign? Well, I'm still thinking about that, but it, too, would have a focus on improving and protecting an existing community, instead of on personal profit and glory. It's a different type of adventure.