Does Leonard Part 6 count?

NOW you're getting it! ;)

Yes, but it's a sequel to a film that never was. They touted it as a sequel to the original Wonderful Wizard of Oz but I always found that laughable.

I can't say the concept stretches across an entire genre of film, or is even large enough for a discussion...but I have to admit I'm kind of tickled that I thought of pretty much almost all of them (nice reference, Tref).

It's funny, just saw a Rocketboom on sitcom spinoffs and shows that are "in the same universe", another weird sequel-ish concept.

Never say Never Again - Sean Connery as an older Bond in a remake of Thunderball?

How about the original Casino Royale - also with an aging Bond coming ut of retirement and predating the true "Bond" films?

Or the original Star Wars?

Oooh, good call on Star Wars. Pick up in the middle of a battle scene, many Bothans already dead.

The Sum of All Fears was written as a sequel to Clear and Present Danger but the movie takes place somewhere randomly before it. There is no established movie canon where TSoAF fits in yet it's part of the series.

Does that count?

But then isn't every movie that starts in the middle of things or references events that happened prior to the movie but aren't shown a sequel to a movie that never was?

I'd say not really. Usually the basic formula is: status quo, something out of the ordinary happens, characters react, resolution.

As in - Imperial rule, a ragtag band of rebels forms and starts crap, war, resolution. We enter that story in the middle. Which is awesome.

Of course this is from a simplistic view of storytelling, but again, we're talking about movies.

The Bond films are definitely part of this weirdness. There are references to the other films sprinkled thinly here and there (particularly in the Moneypenny interactions), but for the most part, they are their own movies. NSNA and the original CR are the oddest ball examples in a standout series that no other film franchise has been able to match.

In the larger category of Sequel Weirdness, Bond gets a place of honor.

Well, if starting in the middle of something qualifies, then Indiana Jones (what adventures/exploits led to that start) and Saving Private Ryan (a lot of unfilmed stuff leading to the landing of LSTs at Norandy) and Pirates of the Caribbean: Curse of the Black Pearl (all of the unseen/unexplained goings on with Jack Black that got him onto a sinking dinghy).

I'd also toss in The Return of Captain Invincible and The Adventures of Buckaroo Banzai Across The Eight Dimension.

The former tells the story of a superhero who comes out of retirement (and suggests a long history of the character), while the latter is presented as one in a series of adventures.

...and is a musical with songs by Richard O'Brien who also wrote Rocky Horror.