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I'm just sitting and wondering whose Casey Junior's sensei for the Mutant Mashup Au (created by me and @possumsarenice).
Tbh Future DaVinci would be the obvious choice, being the oldest and having really good ninja skills makes him good mentor material. In 2012 canon Leo is shown being good with kids all things considered so that would extend to DaVinci. Omg I just remembered Chloe. What if he trained the two and they grew up as siblings?
It'll be way more heart wrenching once Junior has to leave them behind and jump through the time portal. Chloe most likely died minuets before he did too, Junior probably didn't even realize and most likely never will. (Sorry Junior, but the turtle curse decrees that I must bestow the trauma)
Moving on before I ramble about that idea. Future Blue is obviously also a sensei, though in this au he'd probably teach Casey Junior how to be evasive compared to DaVinci, who'd focus more on attacks and techniques. Blue and DaVinci probably taught him together and thus, would both be called sensei. To avoid confusion, I think Junior would call DaVinci, Master, and Blue, Sensei.
We didn't get much info on future Splinter in the Rise movie so he most likely passed away. However, in the Au, both Yoshi and Lou would've been alive long enough to have met Casey Junior as a baby. They both died before Junior could remember them properly, but the turtles, O'Neil's, and Casey made sure to tell him stories about them. Junior calls them Grand Master's Yoshi and Lou.
Of course there's another way of looking at things. Maybe DaVinci died before ever even meeting Junior and is either called Master or Uncle as a sign of respect.
Future Sanzio's also a sensei candidate, like it or not he's a softie for little ones, and would totally want to teach and take care of Casey Junior. (Honestly though, Zio and Future Red act as Junior's dad's more than sensei's). I think Sanzio also wants to protect Junior because Casey was his best friend, right next to Cassandra, so the attachment was unreal.
Speaking of Future Casey, I wanna say he died midway through the invasion and spent a good portion of time being the world's best uncle. He'd definitely encourage Junior to commit as much chaos as possible. He was the one to take care of Junior most often because Junior was all he had left of Cass after she died. Even if the kid wasn't actually her's, Cass saved him for a reason and Casey'll be damned if he didn't make sure Junior lived through the apocalypse like she wanted. Casey probably died sacrificing himself to save a bunch of people, maybe alongside DaVinci.
Future Niccolo and Purple are terrifying, especially when making weapons. They always made weapons. It was a necessity honestly, but those two had a blast making things that blew up, consider it a coping mechanism for both of them. That never stopped, even after Nico transferred his soul into a robot he designed. (The fact that 2012 Donnie became a robot as an adult is canon has me very concerned. Especially since Red did the same in this Au).
As a (rather painful) note: Nico always knew he would become a robot in the end. Even if he was the second to last Hamato standing, Nico would've done everything in his power to stay alive. This is because he knows he needs his family and couldn't bear leaving them even if it meant he could live on and watch over them as a spirit alongside his father.
Of course, Future Simoni is up there reigning as best uncle, not the favorite because that honor goes to Casey. He did a lot of crazy stuff and gave the Kraang the most grief in fights. Simon wasn't the most powerful but he was the most slippery and pissed off the Kraang the most. The Kraang hated that Simon fought like it was nothing and took that as an insult to their power.