In a lot of ways, your average D&D adventurers are a lot closer to the characters in a Western than the protagonists of a bog-standard fantasy novel. Your less scrupulous ones would jump at the chance to find a stash of outlaw gold buried in a distant cemetery, battling through rivals and the continuing war across the countryside; others would be willing to take a minuscule reward to help defend a village from murderous bandits. What I've read about Old School modules and play style seems to mesh quite well with the Western genre. Inspired by Robertson Games' Weird Western game and the Wild West OD&D postings from odd74, my main project on Fistful of Coppers is to create a simple, cinematic Old School-inspired game to emulate both traditional Hollywood westerns, film and TV, and their Italian cousins.
I'd like the classes to be relatively broad, but there will still have to be a lot of alteration from standard White Box characters. Since the primary form of combat in a Western is gunslinging, the Fighting Man class will definitely have to be expanded to include many more ranged options without excluding players who want their characters to be a Bud Spencer-esque brawler, a knife fighter or a displaced samurai. Alongside the traditional Fighting Man, a Ranger or Outdoorsman class fits the genre very well. Its Tracking and Favored Foe abilities easily represent bounty hunters, Indian braves, mountain men and other frontier folk.
The other classes don't come so easily. The Scoundrel or Maverick class is definitely a necessity and could easily cover a broad range of thief-type characters, from professional bank robbers to gentlemen gamblers. Perhaps there are a set number of Specialties that a Maverick could pick from, including lockpicking/safecracking, explosives, smooth talk, sleight of hand and other criminal skills. A Sawbones or Doctor class, most likely inspired by Mike D's Medic, would make sense, but is very narrow in its application.
At this point, these are only my first steps towards the eventual final product. I'll be posting my progress as ideas become more solidified.