In my opinion, I feel we need to see the joker again in the third installment.
Here is a British actor that you may well know. Micheal Sheen.
HE is a great actor so I'm sure he can pull off, he has demonstrated in many roles that he can completely and believably immerse himself in a character.
Physically, if he's costume in done similar to the traditional joker costume unlike the costume heath ledger has(which wasn't bad, it worked well) but i believe if micheal sheen wore the whole suit he would look great.
The make up would simply make him look spot on.