Hogan's Heroes/WMG

Klink and Schultz are Resistance leaders
It would totally break the mood to connect them with the Valkyrie plot, but Klink is definitely the same sort of officer who was involved with previous anti-Nazi groups. As for Schultz, there are White Rose connections that couldn't be proved.
 * And apparently "Nimrod" (a super British spy) is in camp and able to slip Hogan plans for a jet fighter. That means it's Klink, Schultz, Heidi/Helga or Bulkhaulter. Given Bulkhaulter's position on the general staff and expressed dislike of Hitler and the Gestapo, it's not a long leap to say it's him; but the sheer amount of chaos that Schultz ignores makes it even more probable it's him.
 * The Snopes.com Messageboard had a thing where people were writing obituaries of fictional characters, and Klink's was to this effect. It helps that Klink's background lines up pretty well with that of the Valkyrie plotters.

Schultz is a deep cover British spy. From World War One.
See above. He ignores a LOT of stuff that Hogan does, including references to tunnels, multiple women, other resistance agents in the barracks, and the off explosion that is clearly the fault of the prisoners.

"Sometimes I have to be on our side," he says one time when cracking down on Hogan. It's not because he's a loyal German soldier; it's because he's preserving his cover. The times when he is overtly "on the Nazi side" are when Nazis other then Klink are in camp, such as Hockstetter (who's actively looking for spies).

Colonel (and logically later General) Robert Hogan is responsible for the Impossible Mission Force.
He saw how effective a five man band of variously skilled saboteurs and espionage experts was. When the Cold War froze over, Hogan suggested setting up multiple versions of his 'heroes' for the American government and possibly nations allied with us. It's possible that the original team did consist of Newkirk, LeBeau, Carter and Kinchloe.

In the present day of that 'verse, the Heroes have their own long section on TV Tropes' Crowning Moment of Awesome page for World War II.
Self-explanatory, really.

Someday Hogan and Klink will join Schultz in the toy business.
It's been established that Schultz owns a toy factory in Bavaria. It's also established that Hogan's a hustler. It's easy to see a scenario where Hogan realizes that it'll be Babies Ever After back home and makes an arrangement to distribute and market Schultz toys in the U.S., probably in exchange for an ownership stake. Klink, meanwhile, would take any work at all in the West over returning to Dresden and life in Soviet-controlled rubble.
 * Working for the man who played him like a flugelhorn for three years?
 * What, you think Klink is strong-willed enough to turn down a job offer in a postwar economy? He'd jump at it like Schultz going after a strudel.
 * They even hinted at this in the show! Once, Klink was talking about what he would do after the war, stating that he might get into office work. Schultz spoke of going back to his toy factory. Klink contemptuously asks "What makes you think the boss will re-hire you?" Schultz smugly states that the boss doesn't have any choice, as Schultz is the boss! Which leads to a chastened Klink offering Schultz his imported cigars with a "Please have a cigar, sir!"

Each season is a reminiscing session with (by then) Lieutenant General/General Hogan and one of the other Heroes.
That's why the series is Out of Order.

Klink and Schultz know all about Hogan's operation, and are waiting until war's end to enlighten him.
No one can possibly as stupid and incompetent and Klink and Schultz. Early in the war, they decided the Allies were sure to win...eventually. When Klink was assigned to Stalag 13, he brought Schultz along; the two collaborated in making life tolerable for the prisoners, in hopes of getting leniency after the war. When Hogan came on the scene, they quickly realized what he was doing, and chose to turn a blind eye. They documented Hogan's activities, though. Once they were sure the Allies were about to take the camp, they brought Hogan into Klink's office and showed him they could have stopped him, but didn't.

Klink and Schultz walked free after the war.
Following VE Day and the liberation of Allied POWs, the Heroes testified that the camp commandant and the top guard were humane in their treatment and that their own covert operations couldn't have gone as well under the scrutiny of somebody else. Seeing the evidence, possibly including the colonel's own notes from the guess above or information that they were working for the Allies in some covert fashion, the court-martial agrees, and Klink and Schultz are free to go.

Klink knows about Hogan's operations, but doesn't care because Hogan keeps saving him from the Russian front.
As long as neither Hogan nor any of the other prisoners actually attempt to escape, Klink is willing to turn a blind eye to just about anything because Hogan makes sure to bail him out of any trouble that would get him sent to the Russian front.

Hogan and his men discovered a temporal portal beneath the camp
Using this, they went back in time to pre-war Germany and built their camp-beneath-the-camp, then returned to their present time and exploited it fully, pulling off the miracles we saw.