I said that I would tackle a Mattel TMNT line I'm looking at a WWE Scaled line with WWE Articulation for the adult collectors.
For the kids a line similar to Origins in scale and articulation. The sculpt should be what mattel would have done if they had TMNT in the eighties.
The Adult line would be a MOTUC line... not in style unfortunately, but covering all avenues of TMNT, including collaborating with the owners of NT:The Next Mutation.
Let's tackle scale.
Turtles: 5.5"
This is self explanatory. Turtles need to be short for they are teens.
April, Karai, Renet, Irma, and any other adult female: 6"
Adult males would be varied. With 6.5" being the standard. Casey and foot soldiers would be this tall. Mirage styled foot ninjas should be between 5.5" to 6.5" for a reason. The shorter would be foot genin, which would look great with a Tatsu figure. The 6" and the 6.5" can work with Tatsu and Karai or the eventual Mirage variant of Shredder.
Guys like Burne Thompson should be under 6.5" but higher than 6". Vernon and Shredder should be close to 7".
Baxter would be 6" both Mirage and Fred Wolf style.
Teenagers:
IF Mattel pursues Teens:
5.5" for the girls (2012, RotTMNT Aprils.) Like Caitlyn. 6" for Zach and other younger teens (2012 Jones) 6.5" for Carter.
Other characters:
Here it depends:
Ace Duck, Punk frogs can be Turtle sized
Hot spot between 6 and 6.5" sandstorm 6.5"
Muckman, Sargent Bananas 7"
Hothed and halfcourt 8" due to their long necks.
Rocksteady and Bebop close to 7"
Slash close to 6.5"
Pizzaface under 6.5"
Krang: 2" with Bubble walker 4.5"
Android body should be 8".
I can see a few vehicles happening:
Turtle van can get reuse as the Channel 6 van.
Foot and Neutrino cruisers share the same mold for the most part.
Ninja Newscycle and Keno's Pizzacycle.
The Transport Module is unlikely to get a redeco. Nor the Footski.
Some of these vehicles could come with a figure or variant... for example, the Foot Cruiser could come with a Foot Soldier with Greaser themed Gear.
The Pizza Cycle could come with Keno.
Playsets are highly unlikely... I can see those happening for the kids line.
But I'll leave it here for now and make a part 2 for the Origins styled line.