Dafoe was actually a canadate one time in the 1989 version of Batman. Of course at that time I don't see how they could have used The Joker killed Waynes thing, Dafoe was a lot younger then.
I think Dafoe would do great as the Joker, but I just don't see it since he was Green Goblin. He would be a big name if they were looking. It wouldn't be the first time either that a Marvel Movie person crossed over to DC. Bryan Singer X-Men now Superman Returns, Joss Whedon - one time X-Men 3 contender now Wonder Woman, david Goyer The Blade movies now Batman Begins and The Flash.
It might could happen, but I just don't see it!
The Ceb-Man!