WMG: Seinfeld

Kramer sold marijuana out of his apartment.
Kramer really did have a job - if you can call pot dealing a job. It appeared that he made a good profit, even though it also appeared that he smoked a considerable percentage of what he was selling. This theory explains: why he always had lots of money (always in cash, naturally) even though he didn't have a conventional career; why he was always in a daze; why he was always hungry; why he always seemed to be horny; why he was constantly falling over things; why his social network extended far and wide - even though he was an unmotivated doofus; and why he had strange people in his apartment at all hours of the day and night.

The corner of 1st and 1st is the nexus of the universe.
Not a WMG - explicitly stated to be the case by Jerry.
Kramer is a Marauder and the Seinfeld-verse is his Paradox realm
Exactly What It Says on the Tin

The Lopper was Crazy Joe Davola.
The last time we see Joe Davola is in the "The Pilot" when he tries to attack Jerry in the TV studio. We never find out what happened to him. Perhaps he was arrested for his attempted assault on Jerry and sent to prison. There, Joe became increasingly angry and hostile towards Jerry. When he got out of prison, he hated Jerry so much that he started beheading anyone who looked like him. Kramer did say that the Lopper's victims looked like Jerry.

The final episode in fact depicts the characters dying in an airplane crash, and being judged for their sins in the afterlife. At the end, they're sentenced to purgatory to atone, represented by a year long prison term. It is likely that purgatory in this show's universe is represented by living through your life again until you recognize your mistakes and correct them, as they immediately begin to have the same conversation they had in the first episode.

They are actually in Hell
Remember Newman's speech in the finale about "revealing myself in all my glory" to Jerry? He's the Devil. These characters will never get over their narcissism, their mortal sin, which is why they're in Hell and not Purgatory. All the weird things that happen to them are part of their punishment-the finale just made things more explicit (and of course they still don't get it).
  • The reunion episode of Curb Your Enthusiasm quashes this theory, unfortunately. This theory explains a lot of unanswered questions in the finale, like the apparent Kangaroo Court, latent illegal court proceedings and complete ignorance of actual Good Samaritan Laws, though, as well as a police officer arresting four innocent bystanders/witnesses to a violent crime with video evidence, and ignoring the very mugging and carjacking he is arresting them for not involving themselves with.

The Bizarro version of George
At the same time as the events of "The Opposite," shouldn't the Bizarro counterpart of George (his name was Gene) have been doing the opposite of what he normally does? Obviously that is what he was doing. He ordered a tuna sandwich at his diner. Then he lied to his girlfriend about his lifestyle and was rejected for it. He tried sucking up to his boss (yes, his boss; he wasn't unemployed the way George was), but accidentally offended him instead and so got fired for it. Then he moved back in with his parents, at which point he realized that his normal course of behavior is right and his "do the opposite" idea was misguided.

The show they create at the end of the series is Jerry Seinfeld in the real world
Thus Seinfeld and the real world are mutually fictional.

The policeman played by Neil Flynn in The Summer of George is actually The Janitor.
That's totally what he'd do.

Kramer had undiagnosed Asperger's Syndrome.

Jerry lost his rematch with Duncan Meyer in "The Race".
The happy ending we saw was just Jerry's imagination. This explains why Jerry's girlfriend Lois never appears again: she broke up with him after Jerry's masquerade was exposed. This also explains why Mr.Bevalaqua didn't realize that Jerry got a head start, despite said head start being very obvious. You can see Dunca yelling in slow motion,"He got a head start!"

When Kramer took karate, he trained at a Bobby Wasabi dojo.
Which is why they now apparently group their students by age, not skill level.

The video clerk from "The Couch" is Spence.
Before Spence was a token taker at the subway, he worked for a video store.

Tim Whatley is Walt White.
Walt will be put in witness protection, and must assume the identity of a New York dentist.
  • This seems unlikely since Seinfeld takes place in the 1990s and Breaking Bad takes place approximately 2008-2010. But look at my WMG below...

Tim Whatley was cooking meth during Jerry's dentist appointment.
Jerry thought that he was being molested and that Whatley was having sex with his female assistant while he was sedated because his shirt was untucked and they were buttoning up their clothes. The reason that they removed all of the clothes was so that their clothes would not get the smell of meth while they were cooking. The Penthouse letter that Whatley wrote was to throw Jerry off of his scent.

Nina is Bizarro Elaine
We see Bizzaro versions of Jerry, George, Kramer and even Newman.

The gang would not have gone to jail if they had brought Newman with them.
Newman would have had sympathy for the overweight man who was being mugged and tried to get the gang involved in stopping the mugger.
  • Then Newman would have testified against them when they refused to help.
    • Though stopping an armed robbery is hardly a reasonable expectation.

The undercover cop in "The Subway" was Sgt. Doakes.
Because a motherfucker gets surprised.

The whole "show about nothing" thing is a smokescreen. It's all about Jerry and Elaine
Sometimes it's closer to the surface, sometimes less so, but it's always about one and/or the other of them wanting to get back together, if unconsciously.

Susan Ross didn't die.
At some point, Susan realized that getting married to George, and getting effectively manacled to his friends, would be a massive error in judgment. And the envelopes (which sickened, but didn't kill her) was the last straw; either George was too cheap to get good envelopes, or he actually tried to kill her. So, she decided to fake her death, with the help of her parents (who have many many monies). And they went along with the idea of the foundation, (supposedly) giving away the bulk of Susan's fortune, as a way to make sure that there was no way for George to get his hands on it. In reality, Susan, under a new identity, moved to the West Coast. She found a nice, loving partner and settled down to a comfortable life. And, when her parents told her about George going to jail, she laughed and laughed and laughed.

Jerry and George are half-brothers.
Genetically, George's hair is too dark to be Frank and Estelle's child. However, if Estelle had an affair with a dark-haired man their child could get a dominant dark-haired gene. Dark-haired Morty Seinfeld must have lived in the same neighborhood as Estelle and Frank, since their sons went to the same public school. Morty impregnated Estelle about the same time he and Helen were conceiving a legitimate child of their own. Two families living nearby with sons close in age resulted in the sons forming a lifelong friendship even when the parents' relationships soured after Frank and Helen found out about the affair. (The story of George falling off the rope in gym class was either a mere apocryphal fabrication or simply one of his earliest memories of their childhood.)

As a result, Frank so resented George that he constantly antagonized George to the extent of completely ignoring his other son, which is why we never see him, while Estelle was so ashamed of George she just wished they were all as happy as the Brauns next door; however, Morty sweet-talked Helen into reconciling with the silver tongue that got him so far in the raincoat business, thus Jerry has a much more loving relationship with his parents.

George is a closeted homosexual.
While showing obvious interest in being with women, George seems to constantly fail at every aspect of connecting with them, yet can discuss his thoughts and feelings (to an extent) with other men. George's obsession with finding a wife, even one that he realized he didn't particularly want to marry, seemed to be a way of convincing himself of his own heterosexuality and desirability by a woman. George shows constant hypersensitivity over his masculinity (accidentally buying women's glasses, needing to give a disclaimer before saying the word 'fabulous'), panicked over receiving a massage from a male therapist because "it moved", and obviously has latent romantic/sexual feelings for Jerry that he tries not to think about (the girlfriend who "looks just like him."). George has feelings that he simply does not want to face or is at the very least bi-curious.