This is copied and paste from an interview I came across earlier today.
Interviewer: Was he (Leone) thinking about making a last western?
Sir. C. Frayling: Yes he was. You’ve got to read the book. I have the treatment. His last script. It was to have starred Richard Gere and Mickey Rourke. Two brothers in the American Civil War. One is wearing grey, one is wearing blue.
Richard Gere? Man oh man, I can’t even imagine!
With a good director even Richard Gere can be good!
I can’t stand him so I can’t even imagine him being good… 
Even with the Gere Effect, it could have been good. Remember “Days of Heaven”…
Flynn
That story doesn’t really sound good I think… but Leone is a good director so it could have been a good one.
The movie was planned as a pilot for a television mini-series called “Colt, an American Legend”. As Rourke and Gere wanted to make a western with Leone, Luca Morsella (being contacted by Leone before for “Colt”) and Sergio Leone came to the conclusion, that “Colt” was not suitable for Gere and Rourke. So they started writing (getting help from a writer called Fabio Toncelli) and came up with a script titled “A place only Mary knows”.
Leone would not have directed it himself, but would have been the producer and some kind of supervisor. As director they would have picked the son of Fulvio Morsella. The story would have been about two guys during American Civil War: Mike Kutcher from Georgia, whose job is to enrol men into the Union army. The other is Richard Burns, a shady businessman. A third charakter would have been Francesco, a young Italian arriving at the port of Boston during the opening scene. Their adventures would take them from Boston in 1863, to a mining camp in Colorado, to Atlanta, Georgia, where the gold of a robbery is burried inside a unmarked grave - a place only Mary knows, an ex-girlfriend of Mike Kutcher.
“A place only Mary knows” would have been a homage to:
Edgar Lee Masters (Spoon River Anthology)
Ambrose Bierce (A occurrence at Owl Creek Bridge)
Mark Twain (A military campaign that failed)
Stephen Crane (The red badge of Courage)
Margaret Mitchell (Gone with the wind)
Just read that information in Fraylings book…
Did not know that, interesting fact.
Imagin if it had happened! It might have rivaled “Once Up on a Time in the West”!