Note, director Nolan was sitting right there and seemingly didn't correct Oldman's spillage. So, this may be more than conjecture!
This from
MovieWeb.com...
While conducting interviews with the cast of The Dark Knight this afternoon, I got a chance to chat with director Christopher Nolan and Gary Oldman about the proposed third segment of their supposed Batman trilogy... When asked if he thought they should recast Ledger for the third film, or just not include the character, Gary thought it was a good idea...
Oldman then took a moment to think about it some more, "Maybe we don't need the Joker. Because we'll have The Riddler." Yes, Oldman alluded to the fact that The Riddler may in fact be the next on-screen Batman villain.
What of Two-Face you say? Maybe Harvey IS the central villain in the next film, but that doesn't mean they won't set up The Riddler as the headliner for a 4th film by giving him a part in the 3rd. Sounds logical.