He's managed to play just about everyone else — so why not have Benedict Cumberbatch play the Joker? He'd probably be pretty amazing, and a worthy successor to Ledger, Nicholson and Hamill.
And here's Tuis' art of various stars playing Marvel superheroes. Including Bruce Lee as Spider-Man, Arnold Schwarznegger as Colossus, and Rutger Hauer as Thor. More at the link.