Teenage Mutant Ninja Turtles/WMG

See Teenage Mutant Ninja Turtles 2003 for the series See Turtles Forever for the movie

The Kamen Rider and TMNT multiverses are one and the same.
To reiterate: In Turtles Forever, 2003 Shredder learns about the existence of the Teenage Mutant Ninja Turtles multiverse and then attempts to erase all of it from reality. This has nothing to do with Kamen Rider at first glance, no?

Well, there's one single link to it all: Ninja Turtles: The Next Mutation. As much as Peter Laird hates it, the reveal of the TMNT multiverse effectively allows it to still exist somewhere in that cosmos.

With that in mind, remember the Next Mutation crossover episode of Power Rangers in Space? Now that we've linked TMNT to PR, note that Mighty Morphin Power Rangers had a three-part Poorly Disguised Pilot for... Masked Rider. That show, while being a hackjob, is nevertheless a Kamen Rider series, and so part of the multiverse etablished by Kamen Rider Decade.

(And if you bring up the argument that Masked Rider became a Show Within a Show in Power Rangers Time Force, there's still another backdoor: the Rangers' connection to Super Sentai, which leads to the canonical Samurai Sentai Shinkenger crossover in Decade...)
 * Alternatively, the Turtles in Power Rangers in Space are that universe's version of the Turtles, not being connected in any way, they're just Mutant Turtles.
 * But they still wouldn't have existed without the existence of Turtle-Prime in the first place, so...
 * Perhaps it's the multiversal equivalent of a Venn diagram, where two multiverses overlap and create a middle universe (i.e. the way the DCU and Marvel Comics multiverses work in regard to their Amalgam Universe). Although thinking about it too much makes my head hurt a bit.
 * The multiverses each have something that works that way. DC has Hypertime and Marvel has Cross-Time.

The 80's Animated Series of Alvin and The Chipmunks takes place in the same continuity as Teenage Mutant Ninja Turtles.
The Chipmunks and the Chipettes are mutated chipmunks. The last animal the boys touched before the mutagen took effect was the kindly songwriter David Seville, who had helped them off of the road he nearly hit them on. They followed him home, and the rest is history.
 * The orphan Australian girl found the Chipettes after they were recently exposed to the mutagen.

The events of Tales of the TMNT v.2 #58 caused the events of Turtles Forever, or maybe vice-versa.
Issue #58 from the second volume of Tales of the TMNT involved the fate of the Ninja Turtles Multiverse, much like Turtles Forever. Perhaps the Mirage Universe Turtles' encounter with Galatouse somehow in Turtles Forever.

Usagi will appear in the new cartoon
and became something like a tradition in the franchise.

Casey Jones is missing from the second film because certain events from the City at War arc in the comics actually happened here too
That is, Casey accidentally kills a teenage mugger and falls into a drunken spiral that damages his relationship with April before leaving New York and falling in love with a pregnant waitress named Gabrielle. Sadly neither Gabe nor her child survive the stressful birth and Casey returns to New York alone to try and rebuild his life. This also explains why he's mellowed out so much in the third film and while he an April don't appear to be dating, still have a rather flirty relationship.
 * Though they got their relationship back on track by the time in TMNT, where it's becoming more serious between them.

The Turtles (and some supporting characters) represent the Seven Deadly Sins.
The giveaway are the colours they wear.


 * Michealangelo being Orange, is Gluttony. His main shtick is his constant consumption of pizza, which extends even to other parts of the Multiverse as well. In the 80's cartoon, this is done to the extent of putting insane toppings that most would find repulsive on them (such as jellybeans and guacamole).
 * Raphael is Red, and is obviously Anger. Raph is constantly referred to as the most anti-social Turtle of the group, and one that has to constantly find outlets to channel his rage.
 * Donatello is Purple, and therefore Pride. Don takes pride in his vast knowledge of machines. Consider 'Turtles Forever', where his 80's counterpart explained something that worked in his universe (using a wrench as a hammer, building a mini-portal device) only to have his 2003 counterpart constantly dismiss it until he actually sees it for himself.
 * Leonardo is Blue and is Sloth. This one is a bit dodgy to explain, but here goes. Did you ever notice that there are times when Leonardo would just leave the team to their own devices? That he feels that he's not worthy to lead the Turtles, and goes off on his own? That's his fear of constant commitment to ideals talking. Everybody looks up to him, and he's the most diligent Turtle, but it makes one wonder if he goes on these Sabbaticals just to get away from the responsibility of that burden.
 * April is Yellow, and is Greed. But this only applies to the 80's show. April would do anything for a hot scoop, even to the point of holding her boss at ransom in an early season episode just to get a news crew. She'll even put herself in harm's way just to get an exclusive, and gets notably miffed if Vernon gets the drop on her stories. Though, to be honest, Vernon is more representative of this than her, but she has her moments.
 * Irma is Light Blue and Lust. She constantly awaits the day when some handsome guy would sweep her off of her feet, and even ventured into a sewer when she thought one was there (who was actually Splinter, much to her dismay.) Irma has the makings of someone who'd tackle anything that moves, and then some.
 * Shredder is Jealousy (but sadly, does not wear Green at all.) His origin story reeks of this, killing Hamato Yoshi and his lover for the latter spurning him.

Splinter is one of the rats of NIMH
Or at least a descendant thereof. He was intelligent enough to learn martial arts and how to read even before being mutated, and had a much longer lifespan than a normal rat, during the years Yoshi kept him.
 * Unlikely, since NiMH is based on Maryland, US, while Splinter came from Japan. Though there might be a NiMH branch near where Yoshi lived. It's also possible Splinter was taken by NiMH sometime after Yoshi and Shen was murdered. It would have to be in New York since Maryland is miles apart.