I don't see a need for different classes or modern, electronic timers. I'm afraid it would only encourage a race mentality, which is what I'd like to get away from. I'm coming at this from the rendezvous approach -- here are the targets, a hit is ten points, a miss is zip. Add it up at the end of the match and high point wins. A tie requires a shoot-off, or a knife throw, or a... Distances can vary according to the range, and something like a woods walk or a renegade attack can add a lot of fun to an event without a lot of extra work. Most stage set-ups I've seen would also work, just don't shoot it with a timer and include a few targets that will scatter the final scores.
I've set up shoots using watermelon gourds, squash, prickly pear pads, chili peppers, pieces of cork, wooden dowels, charcoal, ice (in winter), and even a skillet from a junk shop set waay out there, because I wasn't sure how much punishment it could take if it was hit too many times. I generally tried to set up a match as fair as possible; for every long range target that favored a rifle, I'd include something to favor a smoothbore. I'd really like to get away from electronic timer and racing the clock. With SASS, NCOWS, Western Three-Gun, Wild Bunch, etc., there are plenty of those types of matches readily available.
Nothing's set in stone at this point, and bumping the time back to 1865 is certainly feasible.