Heath Ledger had so much makeup and there's so many people who are great at impersonating his Joker voice/mannerisms that it would be quite easy to give him a scene or two at Arkham without too much trouble.
Acerider wrote:I he doesn't re-use the joker, but two face might be interesting. He brought scarecrow back for the second one.
I think 'The Dark Knight' was already pretty much about Harvey Dent. Batman and the Joker did somewhat revolve around the character's decline, but while I agree that we didn't get to see much of Two-Face himself (like Darth Vader in Star Wars E3), bringing him back so soon after an apparent death scene might be a bit more than suspension of disbelief could handle.
As stated above the quote, though, we should see some fresh villains. I'd avoid Clayface due to the last Spiderman film, but there's other recognizable ones from the Animated Series (placing them in relatively recent memory) to be getting on with, if they didn't want to resurrect forgotten characters from the comics.
I'd say a logical step would be to throw in a bit of Dr. Harleen Quinzel/Dr. Hugo Strange time at Arkham with the ensconced Joker. The more 'active' victim is a tough call. The Ventriloquist would be good, but it might be a tough sell to a jaded audience alongside Arkham drama. Bane (a proper, intelligent Bane) would probably fill in nicely for action. Just have to think of a way for him to interact with the other villains, which shouldn't be too hard.