Have you read Harold Lamb's books on Tamerlaine and Genghis Kahn? I read them several times when I was a boy and really got swept away. I agree it would be a great subject, but not from the kind of people who just did Troy or Alexander.
Have you read the two volumes of The Last Lion, about the early and middle years of Churchill, by William Manchester? Spectacular biographies, though the fact that he never could finish the third volume, the war years and on to his death, is a true shame.
I feel equally strongly about the first two volumes of the TR bios written by Edmund Morris, The Rise of Theodore Roosevelt and Theodore Rex. The first one, about Roosevelt's early years, up to the day he becomes President, is going to be the basis for the Scorsese / DiCaprio TR biopic. There's way too much there to do much other than his "callow youth," maybe up to the Rough Riders story, yet there's so much more.
If you haven't read any of thoes biographies I'd recommend them highly.