There are lot's of parts that Denzel could play, but he may think he already did this bit in Glory. I think it's too big a story to do right in a single movie. I'd love to see an HBO series, ala Band of Brothers or From The Earth To The Moon.
My actual fantasy is to have the Buffalo Soldiers be part of 3 or four episodes of a grand epic that covers from 1850 (enough before the Civil War to set the stage) to 1920 (enough after WW1 to show the aftermath). I've already got a rough outline if anyone is interested. Main characters are Sam Clemens, Edwin Booth (who saved the life of) Robert Todd Lincoln, General/Governor Lew Wallace (author of Ben Hur, judge at trial of Lincoln assasin conspiracy, Gov of New Mexico who ended Lincoln County Wars, ambassador to Ottoman Empire), Teddy Roosevelt, Nikola Tesla, Thomas Edison, George Westinghouse, Edward the Prince of Wales, Queen Victoria, Kaiser Wilhelm, etc. It's a good list and you can make a great case for these people either knowing each other or being connected through, at most, one other person.