It's a Wonderful Life/WMG

George Bailey dies before the end of the film.
See this post for details.

Mr. Potter is a muggle relative of Harry Potter. All the money that Harry has came from Bedford Falls!

 * So that's where the $8,000 went.
 * He's also related to Colonel Sherman Potter.

Clarence takes George to an Alternate Universe where he was never born.
Clarence is an angel. He hasn't gotten his wings yet (which goes with added abilities), but perhaps he can take people into alternate universes if the situation calls for it. It's not that he changes history so that George wasn't born. There's an alternate universe in which George was never born, and Clarence took George to that universe to see what would have happened had he never been born.
 * Note that Clarence might not know that's what he's doing ("he has the IQ of a rabbit"). It's just that's what ended up happening.

When Potter heard about George being bailed out by his friends and family, he gets so angry that he has a heart attack and dies in agony.

 * It's a nice thought.
 * He...kinda did. An alternate (cut) scene shows him dying of a heart attack, but it was taken out. It could still happen.
 * What's the opposite of personal discontinuity? That seems to be what at least two tropers are doing, deciding that an event that did not occur on screen is canon because we like it so much.
 * That's just Fanon. More specifically, Absolutely Happened.

The "lost ending" for this film aired on Saturday Night Live and replayed on all the SNL Christmas specials is/was a genuine lost ending.
Not quite as nice a thought, but it does fix the "Potter is a Karma Houdini" problem. The classic ending is suspected of Getting Crap Past the Radar; the extra footage may have been filmed just in case.

The "angels" are actually god-like beings that humans cannot perceive.
For whatever reason, George falls into their plans, and if he dies, he will not be able to fulfill them.

When Mary broke the glass after the dance, she wished for George to have a miserable life.
He received news that his father died immediately afterwards--things went downhill from there.

Most of what happens in the world where George isn't born is fabricated by the angels.
Rather than played out to their most likely hypothetical potential, several occurrences in the alternate universe are exaggerated or completely falsified to persuade George of his significance.

The fact that Harry's gravestone in the alternate reality reads 1911-1919 is not a mistake.
At least not a mistake on the on the filmmaker's part. Pottersville is so crappy that when Harry died and his parents ordered the gravestone, those responsible for chiseling in the dates put in the wrong year for either Harry's birth or Harry's death. His parents were upset but the stone was so expensive and Pottersville so crappy they couldn't afford to replace it.
 * Getting the birth date wrong seems more likely than them not realizing what year it was.

Mr. Potter is really Satan
Think about it, Pottersville is basically the 1940s equivalent to hell. He tempts people with what seems to be easy deals for them but with an ulterior motive. Bailey even implied that if they sold out to Potter they may as well be selling their souls.

Prosperity is right around the corner for the Baileys
World War II is coming to an end. The United States is coming out of its economic depression. The prosperous 1950's are about to begin, and people are going to start buying land and houses. George is already an established figure in the local real estate industry, and his business has a good reputation as a lender. He's in the perfect position to take advantage of a market boom. Soon he'll own Bedford Falls.