The turtle's new character designs heavily compliment their personalities instead of being a generic copy of one another and the additional choice to have the turtles as different species instead of just the same species of turtle. Not only does it make their designs much more dazzling and memorable to each turtle, their species of turtle compliment nuances in their personalities.
Leo as a Red Ear Slider. Red Ear Sliders are one of the most common pet turtle around the world and the turtle's original species which seems to fit Leo's probable eventual designation as the leader. Not only that, but Red Ear Sliders are quite flashy, much like Leo's personality. Leo also has a generally athletic body type which works well for someone meant to be all around physically competent.
Raph as a Alligator Snapping Turtle. Not only are they huge, which compliments Raph's position as the Big Guy, but they are known to snap and attack other turtles. There might be a reason why he butts heads with his brothers sometimes. While Alligator turtles don't have teeth, they do have an incredibly sharp beak, which Raph mimics with a visible sharp teeth in his design. Alligator Snapping Turtles are also known to eat almost anything edible, which seems to explain why Raph doesn't seem to mind as much to eat food off the ground. Raph isn't just the tallest but also much stockier than his brothers too, giving him a strongman's build and like most strongmen he has tree trunk limbs instead of defined muscle.
Donnie as a Soft-shelled Turtle. It gives Donnie a reason why he needs all that fancy gadgets and a shell protector, because unlike his brothers who have more protective shells, he has an actual soft shell. Softshell Turtles also have excellent vision. Makes sense why Donnie has those goggles now, huh? Donnie is drawn with broad shoulders and stronger thighs much like a swimmer. Even though we never see him swim on the show, it would still make sense for the most aquatic turtle to have a swimmer's build.
Soft-shelled turtles are also very aggressive carnivores, arguably more aggressive than snapper turtles. Donnie is far more verbally aggressive and starts more internal drama than Raph does.
Mikey as a Box Turtle. Real life Box Turtles are known to be able to hide in their shells from danger, which Mikey also been shown to do, whether for the same reason or for an attack. Mikey also has stickers and scribbles all over his design, enunciating his long dormant creative talent often absent from the other shows who tend to favor Mikey as comedic relief. Mikey is the shortest of the group by a fair amount and has the strongest upper legs which is the ideal body type for a gymnast, perfect for the guy who loves doing sick flips around all over the place.
Box turtles are also quite friendly and are commonly made into pets. Mikey's the friendliest of the turtles.
There could even be a Mythology Gag involved. As stated, Mikey's a box turtle, which can hide in their shells. In the first movie, Mikey was the one who ducked into his shell to dodge an attack, leading to the famous "God, I love being a turtle!" moment.
The boys' personalities are also well-represented in their masks—not so much the colours, but how the ends tie up.
Raph, the rough-and-ready leader, has long and tattered ends that have clearly seen battle.
Leo, the flashy "I don't need swords, I have SKILLS" hero type, has long and flowing ends to his mask, perfect for flapping dramatically in the breeze.
The length is similar to Raph's, but with fewer scuffs and tears. This nicely foreshadows Leo's promotion to leader, as he takes Raph's position on the team without the experience behind it.
Donnie's mask is tied with shorter ends, squared off like pixels on a screen. Perfect for the resident tech genius.
Mikey, youngest of the brothers, also has shorter ends to his mask. His tie somewhat resembles bunny ears, fitting his sweet and energetic personality.
It makes perfect sense for a person as nice and friendly as Todd to mutate into a Capybara. Capybaras have been documented for being extremely social animals, friends to most living things, from cats to actual crocodiles, especially turtles.
Raph's math in Hot Soup: The Game actually makes sense. When Leo remarks that Raph went out on his first solo mission when he was Mikey's age (13), Raph remarks that when he was Mikey's age, he was two years older, leaving Leo to wonder how his math works. While this doesn't make sense at first hearing, Raph is actually referring to the age gap between he and Mikey, which is two years. When he was Mikey's age, Mikey was only 11 and Raph was 13. He probably just didn't convey the message very well.
When Splinter steals the boys' pizza to teach them invisibility and stealth, Leo sarcastically asks if he's teaching them how to "starve the enemy to death". Splinter's response—an uneaten slice of pizza to Leo's face—isn't just a "shut up" gesture, but a cheeky way of saying "Oh hush I'm not starving you."
Why is Mrs. Cuddles, likely a Yokai, a puppet for a children's program? The children's laughs suppress her powers. She's a powerful Yokai who thrives and gets power through screams, but since children are unable to distinguish how creepy she is, their laughs are the only thing keeping her from growing unstoppable.
When Sunita attacks Foot Brute, you might notice that her attacks cause a very loud, metallic clang. On first viewing, it would seem that this is for effect, showing how hard the kick was, but after finding out that her boots were actually part of the Dark Armor the metallic sound of her kick makes a lot of sense.
Splinter is shown to be less of a couch potato in the second half of season 1 onward, most likely due to the fact that he's now aware of the Shredder's imminent return and thus needs to get his sons into tip-top shape to fight, and the idea of losing the only family he has left is also on his mind.
In "Bullhop", it seems weird that Big Mama fell in love with Bullhop's dancing skills. Then the Season 2 premiere reveals that she dated Lou Jitsu who was also a skilled dancer, so Bullhop probably just reminded her of Lou!
Donnie’s fear of beach balls may seem random, but it’s likely because of how common it is for turtles to choke and suffocate on plastic objects.
It's fitting that Meatsweats, the mutant cook that wants to eat other mutants, targets Donatello in their first encounter. Soft shelled turtles are farmed to be eaten more than any other specials of turtle.
While it might just be a coincidence on the animation team's part, Leo is the only one of the turtles who's shown to use a sleeping mask. Red Eared Sliders are sensitive to light and can't sleep in bright areas so it'd make sense that Leo would need a sleep mask to ensure things are dark enough.
At first, it may seem rude and dismissive for Splinter to call the turtles by their mask colors rather than their names, but it makes sense if you think about it like this: parents in big families tend to get their kids’ names mixed up all the time, not because they don’t care, but simply because there are too many kids to immediately tell apart. It’s quite likely that Splinter gave the boys their masks themselves, since it’s easier for him as an old rat to call them by color than by their names (“Orange” is quicker to remember than “Michelangelo “). This is also makes sense when we see "Goyles, Goyles, Goyles" when Splinter/Lou Jitsu called pre-mutated Leo and Raph "Green #1" and "Green #2" before comically asking how he could tell them apart.
Adding to this is that rats are extremely nearsighted, to the point that anything not an inch from their face is just a huge multi-shaded mess. Since Splinter isn’t able to pick out even large details like height or shell design (and it doesn’t help how the identically-green boys are constantly moving around), he’s forced to rely on the brightly colored masks instead.
Each of the turtles display very specific fighting styles based off their weapons and personalities:
Mikey is unpredictable and chaotic, so he tends to jump and flip quite a lot, and is a carefree Cloud Cuckoo Lander, and his Kusari-fundo is well suited for its unpredictability.
In "Todd Scouts", when asked by Todd what they would eat in nature, the turtles pick objects that would seem inedible, since Raph picks a rock, Donnie a twig with leaves, and Leo lunges to eat Donnie. Mikey is the only one who picks something sensical, since he picks berries (however, Todd quickly dissuades him since their poisonous). In reality, turtles do eat those things in real life. A few breeds of turtle consume rocks in order to get certain minerals, and plenty of turtles are omnivorous, so a twig isn't too out of the question. As for cannibalism, more carnivorous turtles will resort to it if the option is open. Mikey is also correct as well, since some turtles eat berries in Real Life.
Mikey also probably picked something that was close to human food since he's a cook, and uses ingredients humans use all the time.
Leonardo's interest in stage magic in "The Clothes Don't Make The Turtle" might seem to come out of nowhere, but given Leonardo's manipulative trickster tendencies, it does make some sense; magic tricks are generally about misleading the audience.
From the same episode, Leo nervously asks Donnie how much he's recorded in addition to Hypno's song. While it makes sense that no one would appreciate being spied on by their brother, Leo has additional reason to worry: if Donnie recorded the music inside of the Magic Townhouse, it's pretty likely he could have caught Leo stealing Houdini's journals on tape.
At first, it might just seem to be for the sake of the plot that Donnie never runs out of supplies and technology despite that he lives in the sewers. However, in “The Mutant Menace”, Raph claims that their family is broke, only for Donnie to be seen counting a large wad of money. Considering that this Donnie is a lot less...scrupulous than other iterations, it’s not unlikely that he’s doing some illegal stuff on the side and using the money for his tech, while hiding it from his family.
Big Mama kidnapping and betraying Lou Jitsu for her own selfish wants right when he proposes makes sense, since female spiders have a tendency to eat males right after mating.
In "Always Be Brownies", the Foot Recruit operates a brownie business under the name Grandma CJ so that she can eventually take over the world. The CJ is a nice nod to how those are the initials for her real name, Cassandra "Casey" Jones, as shown in the series finale.
Raph being compelled to eat the pepperoni mush at the end of 'Origami Tsunami' despite his brothers attempting to dissuade him is fitting behavior for an alligator snapping turtle given that they're known for being opportunistic feeders that will bite down on whatevers in front of them.
In "The Clothes Makes The Turtle" Donnie is the one falling the most into Hypno's trap despite being the most logical and skeptical of the group. Why? The Lotus-Eater Machine Hypno put them in is an 80's makeover montage controlled by a very danceable music track. We've seen in 'The Purple Jacket' that Donnie can be very into fashion and his love for dancing has been a Running Gag the whole series. The illusion matches his interest so well that even after the trap is pointed out to him he doesn't to fight it he's even the only one who doesn't hit the speaker when they smash it.
It seems convenient that Raph could seemingly drown after falling off of the bridge in the Grand Nexus only to turn up to save the day later. However, alligator snapping turtles can stay comfortably underwater for about an hour before they need to come up for air. Staying underwater and swimming a long distance to get away from Big Mama would have been no problem for him at all.
In Down with the Sickness why did the turtles quarantine bubbles come equipped with a cooling foam to break their fevers when they didn't use anything like that on Splinter? Turtles are cold blooded and therefore have a difficult time regulating their body temperatures. If they are suffering from an extremely high temperature then an immediate cool down is absolutely necessary. The brothers know exactly how the flu affects their father and knew from experience that Splinter can handle a fever himself.
Most TMNT properties portray the 4 turtles as blood siblings, who are Happily Adopted by Splinter. In this version, they are a Family of Choice, since they are 4 different species. But with The Reveal of Splinter's Human form being the human genetic template for the 4 turtles humanoid forms, it makes all 5 of them genetically linked, and in a manner of speaking, Truer to the Text for the 4 turtles, while also making them genetic relatives to Splinter.
. As he is part of the turtle's team, it only makes sense for him to take the form of another turtle.