As TMNT 1 from 1990 is my all time favourite TMNT movie, i'd do something like that one.
Characters:
Splinter:
Once Hamato Yoshi, now mutated into a rat.
Personality: Wise, really calm, and always shows how he loves his sons (by buying them pizza or being a listening ear for them incase they are worried or stressed).
Hobbies: Meditating, reading various novels, practising more ninja arts.
Raphael:
Born a human, mutated into a turtle.
Personality: Cares deeply for his father and his brothers to the point he may get into a rage if one of them gets hurt or threatend. He has a temper even to his brothers but not as bad as he has to people outside of his familly. He rather kills than forgives enemies but he does save people, or even cats from a tree.
Hobbies: Weight lifting (body building stuff in general), fighting against training dummies, watching movies.
Fighting style: A more in-your-face fighting style, he is the aggresive fighter that keeps fighting without a second thought. He usually insults/provokes his enemies in hopes that the enemy becomes angry.
Donatello:
Born a human, mutated into a turtle.
Personality: Pretty shy when it comes to outsiders of the familly, he doesn't talk much to anyone but his brothers and father. He is obsessed with technology like tv's, radio's and computers and loves fixing tech stuff or creating tech stuff on his own. He uses rather fancy "smart" words which sometimes confuses his brothers and he tries to talk more 'normal' which sometimes causes comical stuff to happen.
Hobbies: Fixing and creating tech stuff, reading science novels and watching science tv shows and movies, playing video games.
Fighting style: Uses the shadows to his advantage, usually strikes from the back, he mostly dodges enemy attacks rather than attack himself if it's from the front.
Michaelangelo:
Born a human, mutated into a turtle.
Personality: He loves to make jokes and uses witty (childlike) sarcasm to certain situations to make the situation less scary and serious. He cares deeply for anything that lives, even so deeply that he may become depressed if he watches a movie where a human or animal gets even the slightest bit of pain. His weakness is that he cares too much to the point he gets too sad to do anything. But he has a strength aswell, he is the only brother that would go through a fire to even save a fly from dying.
He uses comedy to cheer people up.
Hobbies: Reading comics, watching comedy shows and playing (non violent) video games.
Fighting style: Hops all over the place and runs around a lot, uses his enviorment to his advantage (throwing bricks, uses a lantern pole to do a drop kick etc).
Leonardo:
Born a human, mutated into a turtle.
Personality: He is serious although not as serious as Raphael, Leonardo can laugh at jokes but he doesn't make jokes himself. He is smart when it comes to strategies, planning ahead and adapting to any situation. He, like Mikey, sees the good in anyone even in villians and thinks that anyone can have a change of heart for the better. He is very noble and would love to sacrifice himself for his familly, friends or the world in general and would not hesitate to do so. He takes the lead and always stands infront of his brothers so that he would take the first hit instead of his brothers.
Hobbies: Training his ninja arts, meditating, watching movies (fighting movies usually, or occasionally comedies).
Fighting style: Mostly only counter attacks enemies, uses old fashioned ninja skills and items like shurikens and smoke bombs.
April O'neil:
An early 20 year old woman working for a newspaper company.
Personality: Very curious and always wants to know what is going on, who what is and what anything can do. She usually asks a lot of questions and sometimes acts like a self-made detective.
Hobbies: Writing, reading basically anything and watching reality tv shows.
Irma:
An early 20 year old woman working for a newspaper company.
Personality: Slightly perverted (but in comical way, no extreme ways) and very shy and usually talks about how she can't get a date and how bad she wants to date a man. She is easily scared and gets anxiety rather quickly, which means April will comfort her to calm down.
Hobbies: Writing, reading romance stories and watching romance tv shows and movies.
Casey Jones:
A mid 20 year old orphan, now a runaway street fighter for 'justice'.
Personality: Swears at times while he talks, gives everyone a funny comical nickname (friend or enemy) and he talks a bit too much to the point people tell him that he is blabbering on and on. He never forgives his enemies, but unlike Raphael, Casey doesn't kill he however brings them to the police.
Casey does have a temper like Raphael, very similair.
Hobbies: Fighting crime, reading police comics and books, flirting with random girls.
The Shredder:
Once a ninja from Japan, now a master mind ninja in New York.
Personality: Ruthless, he kills his enemies without a second thought and tortures people who failed him even once to the point where his victims may sometimes become insane because of the pain and fear. He speaks as a leader and acts as if he is the best and strongest man in the world. When someone looks at him funny he will warn them once, if they do it again he kills them.
His only love is his daughter Karai, who is always forgives and he never fears to show how much he loves her, however he won't show any love for her when anyone is near.
Fighting style: Uses a spear as a weapon, shadows to his advantage and various ninja items. He uses surprise attacks and counters, but he always tries to kill his opponent as fast as possible and never plays around.
Baxter Stockman:
A man that works for a science lab, but now a captive for the Shredder.
Personality: A very scared man who obeys the Shredders every command, gets anxiety easily and doesn't shy away from crying in public- even if the Shredders sees it.
He plots against the Shredder in secret to beat him and allies himself with the Turtles in secret.
Hobbies: Fixing tech stuff and making tech stuff, reading science novels.
Karai:
Oroku Saki's daughter.
Personality: Very strict and ruthless, kills without a second thought. She doesn't like it when her father, Oroku Saki, tortures people but she has to accept it eventhough she hates it. She is loyal to her father and does anything he wants her to do wether its thievery, assasination or traiing the foot soldiers.
Fighting style: Uses shadows to her advantage, uses various ninja items and usually strikes from behind. She plays with her opponents to stall because she enjoys fights and mocks people during it.
Minor characters
Vernon: Stuck up and thinks he is the best newspaper writer. April's rival.
Tang Shen: A spirit of Hamato Yoshi's wife and mother to the (the human) turtles. Cares for her kids and husband in the after life. (She is only seen and heard in Splinter and Leonardo's meditions).
Nancy: Runs the orphange and has been searching for the lost Casey jones. She cares for the kids and teens in the orphanage.
Oyuki: A girl from the orphange, she is a rebel and tries to follow in Casey's footsteps but fails at it.
Mom and dad O'neil: Live in an antique store's second floor. They occasionally call April to get some updates on her life.
Story:
Hamato Yoshi and Oroku Saki we're both childhood friends but eventually became friendly rivals as they grew older.
They joined the foot clan, a clan created by Hamato's ancestors and now run by Hamato's father. Hamato and Oroku trained daily and competed in many things together however Hamato was always better than Oroku in every way, then the rival clan of the foot raided the village and Oroku saw his parents getting killed at the early of of 15 which scarred him deeply and got him into a depression.
Hamato Yoshi's father later gave the leadership of the clan to Yoshi at the age of 18 because his parents were leaving on a spiritual trip, this angered Saki.
Saki later betrayed Yoshi and framed him with lies and manipulation on the age of 34. He also killed Tang Shen.
Yoshi, after being kicked out of his own clan by the lies Saki spread, left to New York with his 5 sons (5 is not a typo here
), he tried to make a living there but failed and went into the sewers.
His 5 sons are at the age of 4 and had 2 pet turtles found by Yoshi, Yoshi himself was mostly with the rats instead of the 2 turtles.
Later they all, apart from one of the sons, got hit by mutagen which transformed them into 4 turtles and 1 rat over 4 months.
Yoshi now called Splinter woke up once and saw 4 big kid sized turtles, but his other not-affected son was missing, he searched and searched but failed to find his 5th missing son.
13 years pass and the Turtles are now 17 years old and combat ready for the outside.
Okay i'm done, sorry for the huge story but i really wanted to get my creativity out of the way
Had this building up
Not sure if anyone would read it though but if someone is interested, i would love to tell who the fifth son ends up being.