The original Mirage Turtles don't seem to have a specific characterization in this film, seeming to be nothing but Grim and GrittyBad Asses, until you hear the Shredder call them "The Prototypical Turtles", it makes sense, the Mirage Turtles we see in this aren't exactly the ones from the comics, but the ones from issue #1, explaining why they all don't have much of a consistent characterization.
Also, the slideshow of The Multiverse, showing the grim but popular '03 episode "Same as it Never Was". Seems like it will cause continuity problems, until you see that they were actually trying to fix such problems, since said episode would contradict Fast Forward!
Turtles Forever. Indeed, the best way to say farewell to the Mirage-owned franchise. The Title Drop down the line just made it even better.
Amidst those listed on that page (and then some more) who had direct interactions with the Turtles, the Archie Comics and Usagi Yojimbo crossovers all involved Alternate Universes. Utrom Shredder wasn't trying to destroy the entire multiverse, but only the segment of it which had versions of the TMNT, who all spawned from TMNT Issue #1. (On the other hand, the Power Rangers in Space crossover with the TMNT? Took place in the main Power Rangers universe...)
Also, the fact that The Multiverse can be destroyed by anyone just by killing off the Mirage Turtles. In other words, you don't need any superpowers to be an Omnicidal Maniac...
Technically, they were only vulnerable because Eastman and Laird hadn't finished the first comic yet. Once they did, they followed the plotlines until the end of the series, so while they could still be killed after the series was over, it wouldn't affect the other Turtleverses.
The supposed exaggerated cowardice of the 1987 Turtles is actually Fridge Brilliance. Given their awareness of the fourth wall—they, on various occasions, knew they were in "episodes", and even mentioned that they were the show's title. The '87 Turtles were never afraid where they came from because they were the main characters of the show, they knew they would live. Now in Turtles Forever, they were outside of their show and comfort zone- and indeed could really be killed, due to S&P changes and etc.
Also, the exaggerated cartoonyness of the '87 world and characters. The original series was always cartoony, but the first seasons were actually fairly serious, only to Flanderize and become sillier and sillier as they show went on (Though not to the level they were here): But even after the show stopped airing, the universe continued to get zanier, til' they reached the level they were at in the movie.
Actually, the original show had at least a few episodes that were more zany than what happened in the movie. The only real notable thing that seemed out-of-character was the old toon turtles besides Don breaking down after nearly being killed by Utrom Shredder.
Refer to the previous bullet point.
Also, Old Toon Don was more or less Deus Ex Machina-Man.
He pretty much filled that role in the original cartoon though, so it kind of works out.
It was a bit out of character for them not to take anything seriously too, at least with Leonardo and Donatello.
If you pay attention to the various characters' color schemes, it's plain to see that mutant Hun is the exact same color as the OT turtles. This makes perfect sense because as he came into contact with the mutagen, he was pinning OT Don down to the ground. The fact that he was in contact with someone who was already a mutant when the mutagen touched him may also explain why he wound up looking so drastically different— spikes and all.
Why are the 1987 turtles more goofy in this movie than in their show? Well the 1987 show was shown from their prospective - it's obvious they'd see thing more seriously in their world - while the movie is shown from the prospectives of the 2003 turtles - who would find the 1987 enviroment a bit goofy.
This Troper doesn't think Dimension X is a completely separate universe, "just" another dimension that's attached to the "main" 1987 universe. Like how Marvel's Dark Dimension is not a separate alternate universe, but more of a parasite universe.
Try watching this movie right after Back to the Sewers' last episode. With the Technodrome's rampage on modern New York, you had to wonder...where is the Justice Force?
Why is Bebop's human form shown walking the streets of the 87 TMNT universe?
Probably back in 2105. Wedding Bells and Bytes shows that Cody fixed the time window again, so he probably used it to bring Serling back into the future after the wedding. Also, it's Chaplin. No idea about him though.
The only other universe lost would've been the Raving Rabbids universe. Turtles Forever treats Next Mutation as non-canon, so any universes linked to that are safe.
I was under the impression that they merely ignored Next Mutation and the other black sheep of the franchise. Then again, this was back when Peter Laird still owned the rights to the turtles.
It's implied that the Multiverse is not something you can just see with your eyes, and that their could be other turtleverses that were never explored before. So whether you like it or not, it's likely that Next Mutation was just as much in danger as all the other worlds were.
'87 Shredder is kind of an idiot, remember. Note how he's taunting them with some kind of laughter ray even as he goes into the pimped-out Technodrome.
Put an idiot into a shiny room with thousands of buttons activating so many lethal weapons he doesn't know a thing about. How long until he tries them all? If the pimped-out Technodrome reappears on the surface, that universe's New York City is pretty much assured to be razed to the ground in the blink of an eye, while Shredder is still thinking about the laughter ray.
Splinter fires a couple of missles at a Purple Dragon mutated into a dog leaving behind some green slime and he was never seen again either as a human or a dog. Splinter killed someone. It's not surprising since there have been deaths in the 2003 series before, but still...