Now, Ninja Turtles fans will most likely be familiar with the 2009 film, Turtles Forever and how it establishes that every single Teenage Mutant Ninja Turtles comic, TV show, and movie all exists in the multiverse. the Mirage Studios comics, the 1987 Murakami Wolf Studios cartoon, the 2003 cartoon by 4Kids Entertainment, the 1990s films, the 2007 TMNT movie, the Mutant Turtles OVA, etc., etc.
Hell, even future Ninja Turtles shows and comics even established themselves as being part of the multiverse afterward, like the 2012 cartoon crossing over with the 1987 cartoon or how IDW is connected due to Renet from the IDW comics being the same Renet from the Mirage Studios comics as well as how IDW Leonardo and IDW Peter Venkman of Ghostbusters crossed over to the 1987 dimension, or how in an episode of Rise of the Teenage Mutant Ninja Turtles, Donatello's conspiracy board had the Mirage Studio comics, the 1987 cartoon, and the 2012 cartoon as being “alternate universes.” However, the only Ninja Turtles universes I can think of that didn't make crossover appearances or cameos to even establish as being part of the multiverse are the 2014-2016 Paramount movies or, more importantly, the short-lived live-action TV series, Ninja Turtles: The Next Mutation, produced by Saban Entertainment from the late 1990s.
Now, of course, despite the Ninja Turtles' multiverse being infinite with many, many different Ninja Turtles universes, Turtles Forever didn't show all other universes alongside the ones I've already mentioned. I mean, IDW, 2012, Paramount, and Rise were pretty obvious, but Next Mutation was a show that predated Turtles Forever for 12 years, so logically, it should be shown within the Technodrome's monitor, right? But still, regardless, Next Mutation was only 1 show with 1 season and 26 episodes and even a guest appearance on an episode of Power Rangers In Space, and yet, there was no crossover or cameo appearance from these Turtles (including Venus de Milo. Yes, she exist. Deal with it!) in any future shows or comics at all. Well, after thinking about this, I have a theory as to why that is.
Now, I do know the behind-the-scenes reasons being that Next Mutation (along with Power Rangers and other Saban/Fox Kids properties) were being held by Disney at the time of the 2000s, including 2009 when Turtles Forever came out, and all that, I know. But if I have to guess why the Turtles (Even Venus. Again, real) from the Next Mutation universe have not shown up for any future TMNT crossovers or that their weren't acknowledged or even visited by other universes inhabitants and all that, I would say that maybe, just maybe... that Ninja Turtles: The Next Mutation is NOT part of the TMNT multiverse (who am I kidding? The title says it on the top, ya already knew this) And here's why:
Thanks to that crossover episode of Power Rangers In Space, it's established that Ninja Turtles: The Next Mutation exists in the Power Rangers universe, along with Masked Rider. And Power Rangers, since the show started, does a lot of interdimensional shenanigans like the Rita and Zedd's Dark Dimension, or an alternate dimension where the Ninja Storm Rangers are evil while Lothor and his army are the good guys; hell, RPM was established (I just noticed I kept saying 'established' all that much. What's wrong with me? LOL) as being set in an alternate dimension/universe from the main timeline since that Samurai team-up episode, and Dino Charge is also set in its own dimension as established in the 25th Anniversary team-up episode 'Dimensions In Danger' further establishing that Power Rangers exist within a multiverse of its own (BTW, I don't know if the 1995 MMPR film, the Boom! Studios comics (yes, the TV shows and the comics are separate continuities due to the MMPR comics being set in modern times like the 2010s while the MMPR TV show was set in the early 1990s until now. The 1990s did NOT have social media or smartphones!) or the 2017 film exists within that multiverse due lack of crossovers or connections establishing them as such.
Now, I know some of you may say, "Well uuhh, maybe TMNT and Power Rangers all exist in the same multiverse, right? Every single TV show, movies, comics, game, book, etc., all exist in the same multiverse for sure, right?" and... no, that's not true. See, from what I've read, there's no one multiverse, there's like infinite other multiverses existing within the Megaverse. The Teenage Mutant Ninja Turtles multiverse can't possibly be connected to the Power Rangers multiverse at do to a lack of acknowledgment or crossovers from either multiverse.
So that's why Ninja Turtles: The Next Mutation has not made any crossover appearance or even a cameo in other Ninja Turtles media. Is that Next Mutation is only about... I say 1% or 2% of the Power Rangers universe within its own multiverse, further established in Dimensions in Danger. While the TMNT multiverse never acknowledges the Next Mutation Turtles' existence at all.