Bob once said that if your response to anything results in a small book, you're doing it wrong. However, I have a lot to say on the matter.
If I were in charge of the Turtles franchise, I'd make a few changes, maybe even some drastic ones. Though I don't think they'd be for the worse.
First, respect the audience. The Turtles have been around for more than 25 years, so we should pay homage and respect the audience that have been following the Turtles in one form or another since they were children themselves. Poke at the nostalgic nere. I wouldn't keep it as corny as the 80's-90's Saturday morning cartoon, but I wouldn't sling profanity around like a Tarintino film. I wouldn't be afraid to draw on subject matter for older audiences, like fearful scenarios, risk of death, or touching on the themes, strengths, and weaknesses of each turtle.
There was a particularly memorable line from the CG movie, Splinter is talking to Donatello. "This home has become like an empty shell. Each of your brothers have strengths and weaknesses. You must learn to be strong when they are weak. For if you cannot learn this, then our family is lost." I'd have the story expose and touch on each weakness.
I would do my best to recreate the art style from the first movie. I'd love to see more characters like Rocksteady & Bebop, Baxter, or Krang depicted in that style. They'd have to be depicted practically. The practical effects always have more weight to them than CG(unless the whole movie was CG). Think back to Farscape and how much more threatening Scarrans looked, or the early Alien movies before they became all swift and CG. Pan's Labyrinth and the Hellboy series are both better off for using practical effects, or at least heavily reducing the dependance on CG. There's some pretty awesome TMNT fan films out there that use practical effects. Look up "Fight the Foot" and "Casey Jones."
Leo's the leader, Mikey's the goofy one, Don's the "smart" one(whatever that means), but they're each more than one-dimensional stereotypes. Raphael was a smart-alec more than anything in the 1990s cartoon. In every other incarnation, he's a complete hothead!
Donatello is smart. However I wouldn't want to make him some super smart genius that can hack a computer blindfolded. I think that's taking it too far. I'd expect him to be about as smart as he was in the first movie when he and Casey were playing around under the hood of a truck. The computer expertise he demonstrated in Secret of the Ooze is questionable. He reads a lot, and learns. I'd want to emphasize the curiousity of his character, and his tinkering nature to understand how things work. He's always thinking, always wanting to know more, almost with a child-like wonder and vulnerability. His dialogue would reflect that, along with the decoration of his part of the sewer lair(and the occasionally totally dorky one-liner).
Michaelangelo is probably the most athletic of the bunch. Raphael is usually brutally strong, but Mike is more of an all around athlete. Yeah, they're all ninjas, so they all have a certain minimum amount of agility, but Mike more than the others. Heck, nunchuku are an aerobic exercise. There's never been a version where he's not the youngest, and his maturity reflects that. It was surprising in the CG movie that he was one of the two youngest that actually got a job to help support everyone. Maybe there's more to him than the others expect of him? He's a very social creature, he absolutely devouers pop culture. Out of anyone, he's probably the most troubled with his place down below in the sewers. Since he's so social, he wants to meet people. His active, athletic lifestyle and boundless energy, I believe, are partially reflective of this. He's also incredibly close to Don more than the others. This is seen in the movies how he chats with him while they fight.
Raphael usually has those anger control issues. Though I haven't ever seen a version where he struggles with it and tries to resolve it. Has he ever looked inward, admitted it to himself and tried to overcome his violent impulses? Okay, he's a ninja, and a turtle, so he's incredibly violent by nature. All his brothers seem to have control over their power. He's incredibly protective of his brothers. He seems to love Mike the most, he's the most "big-brothery" around him what with the noogies and gags. He really can't be the same around Leo as he is with Mike. I don't know if he's even able to relate to Don or if they're too different. So, Mike is his 'best friend.' if any of his brothers is in trouble, he'll go berserk. But the best and fastest way would be to threaten Mike.
Leonardo would be difficult. He's the leader, the responsible one. Perhaps to a fault? What does the stress do to him? Is he close with any of his brothers? Can he afford to be? Does he feel isolation in being the oldest? How does he handle internally the constant struggle with Raphael? I imagine Leo would enjoy reading history and military books and going to museums in his off days when he's not training.
I'd redesign Slash, make him more "Starkiller"esque. Slash is a twisted mirror image of the Ninja Turtles. He's a possible alternate version of them, raised by Shredder. He's like Bass compared to Mega Man. Slash has always been a representation of brawn over brain. Yes, they all know martial arts, so that requires a certain intelligence, but Splinter emphasizes the need to train the mind as well as the body. Do you think Shredder did the same? Could Slash's undiciplined mind even be jealous of the Turtles? Does Slash recognize the similar natures of himself and them? Does he know what he's missing or what he could have had? Does that drive him to hate them? Would defeating the Turtles mean that all the abuse he suffered at the hands of Shredder mean that it was all worth it? It would show that he could do something that some "other" ninja turtles couldn't. It would prove him superior. And I'd keep his Binky.
I'd like to explore themes about finding your place. They're teenagers, so they naturally feel like outcasts. It doesn't help much that they're mutants and live in a sewer. I'd like to explore what it means when you don't have a home or a place to call your own. By the end, I'd like to reconcile those feelings by accepting they are who they are. Even in a gallery of other mutant creatures like themselves, like Rocksteady, Bebop, Tokka, Rhazar, Slash, and whatever else the Shredder can send at them, those mutants can show the Turtles what they Can be if they were NOT who they are. They could be accepted, have a place, if only they gave up and stopped fighting. But would that sit well with them? NO, and that would show them who they are. That would conclude the theme of 'finding your place' and 'your own kind.'
I've never liked that phrase, 'your own kind.' It's usually some sort of divider, some "us vs them" label used to hate based on religion, skin color, or the place of origin. I like how the theme was explored in Mega Man X when Sigma asks X why he chose to fight 'his own kind.'
A second major theme I'd like to explore is what it means to have a brother. It wouldn't be anything dopey like, "I have to, because he's my brother." Nothing like, "becaus he's my brother." That's a cop out. Like the villain in Die Hard with a Vengance. He hated his brother, Alan Rickman's character deserved to get thrown out a window. He said something like, "not liking your brother isn't the same as not caring when some **** throws him out a window." That's a bad reason. Gene Roddenberry absolutely hated Star Trek V, but I loved it. It showed the family relationship between Kirk, Spock, and McCoy. I especially love the exchange at the end, "I lost a brother once. I was lucky I got him back." "I thought you said men like us don't have families." "I was wrong." Maybe not so much the lines, but the moment it created. Show somewhere in the TMNT story, a time when each turtle puts themselves on the line for each other, show the personal fear, but then the trust in each other. One in trouble, all in trouble. No matter what, they give their all for each other. That's not "because" they're brothers, it's what MAKES them brothers.
A "review" of what the next Ninja Turtles game should be.
http://thebestgameneverplayed.wordpress.com/2011/07/16/teenage-mutant-ninja-turtles/
"Fight the Foot" an excellent TMNT live-action short
http://www.youtube.com/watch?v=ldC6krN7PNM
"Casey Jones" trailer, full version viewable online
http://www.youtube.com/watch?v=U-UbqLOLwuE
Yes, people still care. They care a lot.