Out of Context Hell

Apocalyptic Future

The turtles were fighting side by side with Cassandra when Casey Jr was born, and Leo trained him to be a warrior. Casey Jr only has brief memories of his mother from when he was very young, and he was mostly raised by Leo.

The chips on Leo's plastron are probably battle scars.

Mikey was likely the equivalent of being in his 70's. He was powerful enough that he could unlock almost any mystic ability you could think of, but using them took a toll on him physically. The more powerful the ninpo, the more potential it has to sap your energy.

Mikey can open up portals through all of space and time (as opposed to Leo, who can mostly portal over relatively short distances), but it took him a long time to get to that level of ability.

Mikey probably grew a little taller than seen in the movie, but shrank as the use of his powers depleted him. Donnie would be slightly taller than Leo, and Raph would be at least 6'6".

The characters have obviously taken hits and lost some things from years of fighting the Krang, but Leo still does have his ninpo.

They never worked out the specifics for how Raph and Donnie died, just that they lost their lives in the war against the Krang.

Future Leo was mortally wounded in his bleeding side, and that was why he didn't go with Casey to the past.

If season 2 hadn't been cut short

The sudden declaration of Leo as leader wouldn't have happened, and was more of a joke because of how abruptly the show ended. The original plan was for the transition to be drawn out over much of season 3. Raph and Leo probably would have been co-leaders for a while, and the transfer of leadership wouldn't have caused tension since Raph would like to share the burden. There might have been some situations where Mikey or Donnie took the lead, too. Ron never saw the turtles as a group where there had to be one absolute leader.

Karai would have lived for much longer, and been training the boys in person for a number of episodes.

If there was a season 3 / plans they had for future seasons

They probably wouldn't have ever taken a dark turn with the show, but may have gotten into more of an extended plot line after the movie.

The missing sister who was Big Mama's henchperson would probably have been a very disciplined super serious ninja, to the point where she's actually funny. She was going to be named after a female artist, possibly Frida Kahlo, and the turtles would have had to win her back from Big Mama by helping her to see that she had been brainwashed as a child.

Their other missing sister was going to be trapped in another dimension for years, and might have been "a little kooky" from being there so long. The turtles would have split up to rescue their sisters, but there were no definite plans beyond that.

We would definitely see Casey Jr again. Ron would love to see a spin-off where Casey and Cassandra roam the world and fight the Krang and remnants of the Foot.

It was going to be revealed that a Krang spaceship had crashed into the back of the crying titan in the Hidden City. The ship's fuel was the source of the ooze that gives the Yokai their power, and possibly the origin of the Yokai themselves. The crashed spaceship is what drew the invasion there a thousand years ago.

Mikey's powers could have opened the door to some multiverse episodes.

We might have seen some redemption from Big Mama, but then also seen her relapse to her villainous ways.

The Rat King likely would have been the next big villain. Shredder's story is pretty resolved, so they wouldn't have gone back to him, but maybe they could have revived the dark armor. There were no existing plans for Bishop, but once the turtles became better known heroes, he could be a thorn in their side. They would have done a lot more with the Krang.

Donnie probably thinks he's much smarter than Raph, so it would have been fun to do an episode where he got to see how Raph beats him in common sense and emotional intelligence.

The turtles would probably talk to Karai's Hamato spirit when they need guidance.

Season 3 would pick up where the movie left off. They would have launched a new story right away, but the aftermath would have been in the background. We would have seen how they changed as a result of the movie events, as well as how they deal with now being in the public eye as heroes.

Miscellaneous

The 2012 series was very traditional TMNT, so a big goal of Rise was to mix things up. Making Raph the leader opened up a lot of possibilities for both Leo and Raph: Raph didn't have to be angry all the time about not being the leader, and Leo was free to be cocky and fun-loving. It was a breath of fresh air for writers who had done previous TMNT versions.

As for how the turtles take after Yoshi: Donnie got a lot of his cockiness from him, as did Leo. Mikey senses that Yoshi misses his family, and that's part of the reason he always tries to hold the family together. Raph got his courage and sense of duty from him.

For Halloween, Raph would be a kitten, Mikey would be a lion, Leo would be a rock star, and Donnie would be J. Robert Oppenheimer.

Raph's favorite music is R&B, Leo's is glam rock, Donnie's is techno, and Mikey likes boy bands.

When asked about the turtles "favorite" brothers (obviously they all love each other): Mikey is Raph's favorite. As much as Leo gives Donnie a hard time, he really likes him. Ultimately, the three younger brothers all really look up to Raph.

The turtles all had their mystic powers inherently, but they didn't know it and couldn't unlock them without a little boost. Their mystic weapons acted as a conduit.

Splinter has a lot of power that hasn't been revealed, and there's a good chance he can do anything the boys can do if he really tries (which is how he could use Leo's portals). Their powers all would have grown and expanded over time.

Ron really likes fanart of the turtles, Splinter, and April in emotional family situations, as that is the heart of the show.

They didn't really get into romance in the show, but fans are free to use their imagination.

Hueso would never admit it, but he and Leo do have a close friendship.