Joker Knew of Bane's Existence.
Let's just consider it for a moment. It's silly - I'm well aware of that. But just think about the writing of the two movies, the arc of them, and let your mind wander.
Joker: When the chips are down, these civilized people... they'll eat each other.
See, I'm not not a monster. I'm just ahead of the curve.
He's truly ahead of the curve, because in that interrogation room, Joker seems to tell Batman everything that's about to happen in The Dark Knight Rises. They cast Batman out, like a leper. They needed him right now, but when they didn't... they cast him out.
When Joker's dangling upside down at the end:
Batman: This city just showed you that it's full of people ready to believe in good.
Joker: Until their spirit breaks completely. .... when they get a look at the Real Harvey Dent.
Gotham gets a look at the real Harvey Dent, provided by Bane, and Gotham's spirit breaks completely.
Now, I know this isn't going to fly on these boards, and you'll all view me as a total moron, but just consider the argument one last time.
Joker knew a storm was coming. He was aware of an event happening in the near future, and he was having a ball with it. He was putting in his time, having fun, because he knew everything was about to crumble. Now, is this to say that Joker knew Bane personally? Absolutely not. I'd imagine they never saw each other or exchanged words. But I absolutely think that Joker knew that Bane existed out there somewhere, just like Crane knew about Ra's Al Ghul coming to Gotham.
I'd say Joker's last hoorah before the collapse was dressing up like a clown and mocking society, being a mirror of all the clowns he saw on Wall Street and in Washington D.C., and he had his own fun clowning around... because he knew the end was close, anyway.
Just an idea. Have fun with it.
KEEP CALM AND CALL BATMAN