If the Teenage Mutant Ninja Turtles lived in the Marvel universe, Donatello would either work for Stark Industries or Parker industries, happily inventing whatever he wanted at whichever he was at.

Leonardo and Raphael would open a dojo for mutants and humans together (and Raph would sneak out a night and hang out with Daredevil).

Michaelangelo would most likely own a skate shop, or end up becoming an X-men.

April would assist Donatello with his science while learning to control her powers with the help of either Charles Xavier, Jean Gray or Emma Frost.

Casey Jones would hang out with, and eventually become a part of the Heroes for Hire, finding it a nice way to earn money, while doing the right thing and beating up criminals.

Master Splinter would live in a home paid for by his sons, who thought they owed him, out of the city and away from humans, so he could live in peace, and have his sons visit whenever he wanted, without any mutant hating humans getting in the way.

Mikey would quickly befriend almost everyone, from Johnny Storm, to Spiderman, Ms. Marvel, Deadpool to majority of the X-men, including Kitty Pryde, Kurt Wagner, Bobby Drake, Rouge and Gambit, and some of the Avengers, like Ant-man (Scott Lang).

Donatello became friends with Hank McCoy very quickly due to their similarities and Sam Wilson.

Leonardo made friends with Steve Rogers and Sharon Carter, Scott Summers (Raph doesn't understand why), Daredevil (how he knows Raph sneaks out), Iron Fist, Captain Marvel and Spiderman.

Raph made friends with Luke Cage, Jessica Jones, Daredevil, Wolverine, Punisher, and pretty much any anti-hero out there (excluding Deadpool; he hates Deadpool).

April became friends with most of the female X-men members, and a few male ones as well.

Casey is friends with the Heroes for Hire, and that's about it.