I will say a dark and gritty version would actually be redundant anyway as the first movie already managed the perfect amount of darkness for a Turtles film, and incorporated the better elements of both the comics and the cartoon for a fairly pragmatic adaptation.
I personally would dig an out there TMNT with stuff like Dimension X and all the various strange villains showing up in some fashion. Seeing live action versions of them would be fun to see in theaters.
You know what? Screw it, I want a movie with a Baxter Stockman (flyman version), Leatherhead, Slash, and Metalhead villain team as led by Krang.