While the Joker was excluded from The Dark Knight Rises, his plan still had repercussions on some of the character, namely Jim Gordon. I understand why there was no cameo in the movie, that would have been very disrespectful to Heath Ledger and his family.
My reference idea is very simple yet could be very powerful for fans to speculate and debate over.
During the prison breakout scene, during all the chaos panning down a long hallway coming to a stop at a cell cracked open with a flickering light. On the wall written about a hundred times is the word "HA."
Let me know what you think.