Leonardo: Sports pants, arm greaves, and shoulder pads. Mostly for sword protection. I started with him and made him the "default" off of which the others are based, as I consider him the balance between the three of his brothers (i.e. the "Mario")
Donatello: Tall and skinny (following TMNT2012). Least likely to leave the lair, most likely to tinker around the garage, hence the jeans, and finger-less work gloves. Also carries a satchel containing all the gear the turtles take on their missions. Raph and Mikey make fun of him for it, but then turn right back around when he has something they need in the bag (like smoke bombs, mission paramaters tablet, and/or pidgeon puppet). Oddly enough, Donnie takes inspiration from Mikey when building gear for turtles; sure, it sounds stupid when Mikey says it, but it looks really cool when Donnie actually builds it!
Raphael: Short and Stocky (again, following TMNT2012) I wanted to push the stockiness, hence the gun show. Raph's favorite fictional character is Ryu from Street fighter, who he emulates by ripping his martial arts pants, and constantly wearing sparring arm gear. Raph overcompensates because he feels inadequate compared to his brothers: Leo, the disciplined teacher's pet, Donnie, the smart technical prodigy, and Mikey, the gifted but lazy goofball. He's almost as protective of them as Splinter is.
Michaelangelo: Short and Skinny. I know they were all "born at the same time," but I wanted to emphasize Mikey's age and immaturity in comparison to his brothers. Hence he's the "little brother" (Leo-> Donnie, Raph -> Mikey). Mikey wears skate shorts and sandals for obvious (if slightly stupid)reasons. Mikey's natural athleticism translates well to ninjutsu, but even better to extreme sports, which he prefers (much to Splinter's consternation), as he feels it allows him to be more creative. He also finds himself at odds with his brothers' hangups enough at times (Leo's discipline, Raph's rage, Donnie's aloofness) that he longs to find friends outside them, hence their initial foray (which Mikey instigated by sneaking out against Splinter's orders, and the others following him)
Things I think about way too much. (also, things I should make a fan comic about. Hah!)
What do you think? Leave a comment below!