See? Told you.
Absolutely perfect for the role.
His face is already the right kind of "triangular" shape. Whiten his face, greenify his hair, add on a small prosthetic to make his nose more pointy, pin back his jug ears and then a very slight elongation of his chin into that exaggerated comic book "pointy chin" with CGI or make-up.
Before:
After:
Genuinely, 90% of that "after" image is just changing the colours and the very slightest touch of stretching his chin and nose into a point.
If I can do that with a shit paint program in five minutes then Hollywood could make him look identical to the comic book Joker. No problems.
Chris Eccleston is the Joker.
No, really, he is.
Or, at least, he really, really should be.
p.s. the more I look at the two, the more I'm convinced he actually looks more evil in the "before" than the "after" picture.

"Words ought to be a little wild - for they are assaults of thought on the unthinking."
- John Maynard Keynes