Baron Ungern Von Sternberg, the deranged Russian nobleman and mystic who took over Mongolia and ruled it for less than a year.
Nestor Makhno, the Russian revolutionary who founded an anarchist army and fought both the White and Red armies.
Phillip Seymor Hoffman
Simo Hayha the Finnish sniper who sprayed out masses of bullets what ripped threw the bodies of more than 500 Russians during the Winter War.
Ze'ev Jabotinsky the Zionist leader who formed Jewish defense groups in Russia and later went on to create the "Zion Mule Corps," a Jewish legion of the British army.
And on the humanities side, J.G. Ballard, pioneer of dystopian fiction and neo-surrealist literature.
Dr. H. H. Holmes (the 1890s serial killer [or mass murderer depending on your definition]). There have been several talked about projects with Dicaprio and Depp attached but nothing has surfaced so far.
Alexander the Great- I think he'd make for a great biopic. (I understand Oliver Stone considered making one, but it never happened.)
From the other thread- I'd love to see Gore Vidal's Burr adapted as an HBO miniseries. While the title character's the fulcrum of course, you'll see famous Americans from Jonathan Edwards to Davy Crockett as well.
Recent obituaries have me convinced the life of Aleksandr Solzhenitsyn would be great miniseries potential (particularly if they didn't lionize him too much- he could be an ornery and very inconsistent person).
A movie about the William Desmond Taylor murder or perhaps the Fatty Arbuckle trial- something that really shines a light on the Silent era- could be good as well.
Leonardo Di Caprio
I'd like to see a movie about Otto Friedrich Gideon Sundback, the Swedish-American inventor who perfected the modern zipper. My version would be sort of like the movie Tucker: the Man and His Dream, where Sundback's vision is threatened by the big American button manufacturers. So it probably wouldn't hew too closely to strict historical accuracy. Sundback would also give all his rousing inspirational speeches in a pea soup-thick Swedish Chef accent.