I think that TDK was implying that Joker and Batman would definitely meet again and that he would always be the arch-nemesis. "You and me... I think we're destined to do this forever." If there was a really good role for the Joker to return, i would be for him being recast, since there is already a precidence of recasting with Rachel Dawes.
Holmes, though, didn't own the character to such critical acclaim that people spoke out in horror when the character was recast.
Yeah, but, again, the Joker is an inherently chaotic character. He doesn't even remember his past consistently from day to day according to Alan Moore's The Killing Joke
, and he certainly gave different accounts of his past from scene to scene in the film. So I think the character allows for another actor who might even have a radically different interpretation.
Ledger's performance was amazing, and he deserves an Oscar for it. But this is a character with six decades of history who has been played by at least six actors that I know of. It's not a character about which there can ever be said to be one definitive interpretation.