I look at various comics like Infinite Crisis and Justice and the villains have these huge elaborate plans but they don't include the Joker because he's too crazy. In the end, Joker always takes it personal and kills the enemy leader or something. I think that, if he were in this movie, Bane would have survived his encounter with Batman.
Batman would have been like, "Your broken, Bane. Don't ever cross my path, again." and he would have left Bane cringing in fear at the thought of facing Batman. Then, Batman would have left.
Joker would have stumbled upon Bane as he was recovering and said something like, "I wanna know how you got your scars" or "Let's put a smile on that face." and killed Bane.
Joker is actually in love with Batman. It's true. Google it. Batman is the reason Joker does what he does. In the comic "The Dark Knight Returns", Joker was pretty much a vegtable until Batman came out of retirement. Then he couldn't wait to see Batman face to face and pick up where they left off. Bane broke Batman, almost killed Batman, and to top it all off, he pulled off his plan. Joker was trying to show that the people of Gotham would turn on each other like animals in the second Nolan film. Bane showed that people would turn on each other halfway throught TDKR. Joker wouldn't let Bane outdo him and live.
One way or another, the Joker faces Bane and Bane doesn't make it out alive.