He would have been absolutely huge given the chance.
I dont think he wanted to be huge. He wanted to do roles that were challenging. He'd said that he never got offered the lead roles in blockbusters anymore simply because the studios knew that he'd refuse to do them. He probably only did The Dark Knight because he felt he could something with the role of the Joker, and not because it paid well or because it was high-profile. To be honest, reading between the lines, I don't think he would ever had done 'A Knight's Tale' if he could've rewound the clock. He'd worked hard to disassociate himself with that kind of light-entertainment type film, hence one of the reasons why he took the part in Brokeback Mountain and then Candy.