I'm sure they'll find another gem to fill the role in the future. I wasn't sold on Ledger as playing the Joker initially, nor was I sold on Christian Bale for Batman Begins. A British actor playing a classic American icon? Yeah, right! Glad I was wrong.
Since Batman Begins came out, I've been looking into his other roles (3:10 to Yuma, Reign of Fire, American Psycho). He's a pretty versatile guy.