Maybe I'm being blinded by my love for crazy women, but I think she has a decent shot at a TDKR appearance. A brief appearance is probably more likely than a large role. Perhaps similar to Scarecrow in the TDK, Two-Face in Batman Forever, and Karai in TMNT (yes I went there!). Let me explain my ramblings:
Scarecrow in TDK - Apprehended in the beginning of the movie. A residual element from the previous film. As Casey Jones would say: "Leftovers!"
Two-Face in BF - Full back story not seen, but discussed by other main characters and newscasters. In addition, an intro scene featuring a standoff opposite Gotham's police.
Karai in TMNT - Shreddar defeated, the right-hand girl takes over what's left of the gang. Lacks the vision of the original leader, but still holds things together.
Just my thoughts. A scene in the beginning of the movie could be useful in establishing Batman's dilemma of basically being caught in the middle of the police and the violent criminals he still intends to take down. Harley could be utilized here, as she's a new villain, but at the same time couldn't be described as "random".
And what a trilogy...Joker puts his stamp on the end of the first movie, all of the second, and the beginning of the third.
