Many consider Batman:TAS as the defacto interpretation of the character, and it's for good reason. The action is great, the characters are well-realized and the stories have proven themselves timeless. Ben Affleck doesn't have to live up to Christian Bale or Michael Keaton -- he has to live up to The Animated Series.
Given that it's such a popular take on the Bat, it only makes sense that fans might try to find an underlying theme throughout the series. Well, besides just "Batman gets in a jam and punches his way out." According to this theory, each villain represents a different shard of Bruce Wayne's fractured psyche, still traumatized after witnessing the murder of his parents as a child.
Going down the list, each member of the rogue's gallery shakes out in a logical way. Mr. Freeze is Batman's cold heart, which explains why Bruce Wayne can't maintain a longterm relationship. The Penguin mirrors the intoxicating and corrosive power of the Wayne family fortune. Two-Face is the stand-in for the push and pull between Bruce Wayne and Batman. The Joker reflects the insanity and chaos of Bruce's new world, brought on by the meaningless deaths of his parents. And Condiment King, uh, represents the brutal Sriacha shortage in Gotham City.
The theory goes so far as to suggest that none of these villains actually exist, and that Batman's adventures are all completely imaginary struggles against his own psyche. It's a hard pill to swallow, but sooner or later you'll have to accept that this cartoon isn't real.
It's always seemed strange that The Joker in The Dark Knight seems to hold all the cards, so to speak. Yes, he's a maniac on the level of the guy who doesn't switch train cars when someone poops on their seat, but at the same time he's always one step ahead of an entire police department. Either The Joker is incredibly lucky or everyone else is incredibly dumb. Or he's been in on the joke the whole time.
Remember the heist that opens The Dark Knight? It's a pretty neat sequence, watching an intricate plan executed, and then several of the thieves betray each other and also get executed. As the crowd thins out, one goon points a gun at the disguised Joker after suspecting him of betrayal. The clown corrects him: "No no no, I kill the bus driver" -- as though he was correcting someone on the script, implying that he was aware of the sequence of events following up to that point. It's a punchline shared between the audience and its self-aware clown. The Joker knows he's in a movie, and he's performing for the viewer.
It's the only way this could really work. After all, the Joker says it himself:
Well no, he doesn't look like a guy with a plan. He looks like a lunatic who's along for the ride, doing things off the cuff that still seem magically prepared ahead of time by a cast and crew working around the clock. Clearly The Joker is in cahoots with Christopher Nolan. I wouldn't count Michael Caine out at this point, either.
Need more proof? Consider all those computer screens in Bruce Wayne's penthouse.
Hacking thousands of cell phones to build a giant 3D map of the city seems like a play on the familiar theme of technology vs. privacy vs. security vs. your browser history, but that might not be all that's going on there. It could be that Lucius Freeman is so pissed off about the wall of monitors not because it violates the constituational rights of almost every Gothamite; rather, it's because that breaks the illusion for him. Now that he sees cameras pointed in every direction, he realizes the facade of his own reality, that his life is just entertainment for the masses. You'd quit your job, too.
When we get to the climax, the Joker is actively trying to let Batman in on the joke, attempting to get the Dark Knight to finally kill his archnemesis. This isn't because The Joker is suicidal, or that he wants to break Batman by destroying his moral code, but he wants to make him see that death is trivial, nothing more than a plot point for a movie audience. In the end, The Joker fails to make Batman see the light.
Between cackles, The Joker prophesizes that the two of them are "destined to do this forever." And they are, because The Dark Knight is a movie that will be watched over and over, with the plot always going the exact same way, Batman always just barely triumphing over The Joker.
Just because The Joker doesn't seem to have a plan, it doesn't mean he isn't following one set by someone else.