I like this thread just because i can talk about the movie without having to put SPOILER!! marks on everything i say about The Dark Knight.
..but anyway, id go with Joker returning for a brief time, even if its just for his trial, since his character didnt seem to be over yet. He wouldnt even need the make-up.
Id also bring in BANE. To me, hes the most menacing foe left to face. Cunning, brutal, and willing to cross the one line Batman finds himself unable to. His own career was even inspired by The Dark Knight.