Okay so this will probably just sound insane, but I think that the Narrator from Fight Club abandoned the Tyler Durden persona and later on took on the alias of The Dark Knight's Joker.
You're probably all sitting at your computer thinking I'm just as crazy as Tyler and The Joker combined. Now keep in mind that most of my evidence for this comes from the Fight Club novel by Chuck Palahniuk. It's not too different from the movie but please indulge me for a moment, if you will, while I give evidence for this claim:
1. Tyler Durden lives in a city that remains unnamed.
The primary consensus on the internet is that Fight Club takes place in Wilmington, Delaware. This is because of the real-life locations of the credit card companies they blow up all being in Wilmington. Also, according to the 1990 Atlas of the DC Universe, the fictional geographical location of Gotham City is in New Jersey, across the Delaware Bay from Metropolis. This places Tyler Durden less than 100 miles from Gotham City and very close to a certain mental institution.
2. The Narrator and The Joker both do not have real names.
Other than The Joker having the alias "Joe Kerr" (hehe "Joker" get it?) his name is never given. As for The Narrator, his name is never given in the book or the movie either. Outside of the movie/book however The Narrator is sometimes referred to as "Jack" because of the first-person magazine articles. But it is never said in Fight Club that his name is, in fact, Jack.
I would also like to add that Chuck Palahniuk is currently writing a sequel to Fight Club and has said that The Narrator's name is Jack. This doesn't mean much because given The Dark Knight events, his name wouldn't have been revealed.
3. The scene from Fight Club that screams "Joker."
I sadly, could not find a decent video of the scene. This poor quality video does capture what I am trying to say here though:
Again, sorry about the poor quality.
The Joker would do what Tyler did in this scene: provoke a physically bigger opponent while laughing in their face. This scene is very Joker-esq, it shows that Tyler can take a beating and that he is insane. Batman beats The Joker up every time they meet. Have you ever wondered how The Joker, as scrawny as he is, could just keep coming back at Batman? Batman, a man who likely puts people in the hospital, crippling them permanently, every night. But Joker keeps coming back for more. Why? Because his time in Fight Club/Project Mayhem conditioned him and possibly messed up a lot of his nerve endings so he doesn't feel the same levels of pain that an average person feels.
4.The Joker and Tyler Durden have (not the same but) similar motives for what they are doing.
It can be said that The Joker's motive was to simply wreak havoc for the sake of wreaking havoc. While Tyler was trying to cause mayhem for a broader purpose, to free the people from the confines of material possessions. I think it's possible that after all he and Tyler experienced, The Joker was created to show that people care about the wrong things; essentially that Tyler didn't take anything far enough. To The Joker, Tyler was a failure, he couldn't even completely take over the mind of The Narrator. The Joker got rid of every rule, told nobody his plans, and his followers still didn't ask questions. Which brings me to my next point:
5. Their followers are mentally unstable and cannot think for themselves.
This one really speaks for itself. Clearly everyone in both groups was brainwashed and mentally weak. Some of them could have been from Project Mayhem also, after all the first rule of Project Mayhem is you don't ask questions. So, the original members of Joker's crew wouldn't think twice if their leader started dressing as a clown. Down the road they might, but The Joker wouldn't care, because The Joker has no rules.
6.They both have a knowledge of homemade explosives.
It never says how Tyler just knew what he knew about explosives, but I'm sure he could have looked it up or read books, the same way anyone else would. Only then does this explain Joker's knowledge of explosives.
7. The Scars.
In the Fight Club book, early into when they start doing Fight Club, The Narrator ended up with a hole in the center of his right cheek. Throughout the book the hole got bigger and bigger because of all the fighting, it just wouldn't heal. Then near the end, on the roof of the building he shoots a hole in his left cheek. The page reads as follows:
The bullet out of Tyler's gun, it tore out my other cheek to give me a jagged smile from ear to ear. Yeah, just like an angry Halloween pumpkin. Japanese demon. Dragon of Avarice.
Sounds to me like The Narrator has the same unique Glasgow smile as The Joker.
8. The Narrator got thrown in a mental hospital.
At the end of Fight Club we leave The Narrator while he is in a mental hospital, convinced that his therapist was God, and the orderlies were angels. With The Narrator being in Wilmington, Delaware this places him extremely close to the famous Arkham Asylum. The Joker in other media has broken out of Arkham Asylum before, surely he could have broken out in the Nolanverse between movies, of perhaps during the events of Batman Begins when prisoners were escaping in massive numbers. Nevertheless, this explains how he made it to Gotham.
I also think that while the Joker was ripping off other Mob Dealers he came across some early version of the Clean Slate program, or simply put a gun to someone's head, forcing them to erase him from existence.
Now I'm sure that this theory has some holes in it. You are all welcome to post any issues that you may have with this theory in the comments section and be sure to answer the poll and like us on Facebook!