Hmmm, that'd definitely be a way to sell more tickets - a lot of people who might not normally go would probably want to see if the "new actor" did as good as/better than Heath (probably the same people who went to see TDK because of the circumstances surrounding his death). It would be a gutsy move on Nolan's part to recast the role, though. I imagine there would be a lot of people who believe the role should only be played by Ledger and who would be upset by a recast, likewise it would be a brave actor who would take on the role - having to walk the line between impersonation and taking the character too far from what was established in TDK.
I actually wouldn't be at all surprised if Ledger played an important, albeit minor role in the next film through the use of archival footage and digital manipulation, much in the same way Brando was featured in Superman Returns. That would allow Nolan to still make use of the character while at the same time not angering fans with a new actor.