Some juicy tidbits from an interview with TMNT director Kevin Munroe:
Munroe met with Turtles co-creator Peter Laird with his Issue #1 of the original comic series in hand, and Laird's involvement continued from there and remained a major presence throughout production. As far as he was concerned, if Laird was happy with what they'd done, Munroe felt confident that they'd done right by the longtime fans, which was one of the two biggest influences on sculpting the film in general terms: make it something that is appropriate for the younger audiences and maintains who the Turtles are to tons of fans who've been with them for 20 years.
When asked about live action versus CGI, Munroe is confident the movie as it is couldn't have been done with the hyperkinetic speed and impossible camera angles you see in the film. Interestingly, he also revealed that they "re-created scenes from the first movie with the CGI models and in their opinion, it worked and moved better". He later added "there's also only so much expression you can get out of rubber suits controlled by servos, even though I love what the Jim Henson people did in the previous movies". As for the inclusion of those test scenes on the DVD, they're looking in to it, but there may be rights issues involved that'd prevent it from happening.
TMNT was the last film Mako worked on, and according to Munroe, he was gracious and an absolute pleasure to be around. His first meeting with the legendary Japanese actor was actually in a restroom, while Munroe was washing his hands and Mako came through the door beaming "Kevin, so nice to meet you!" Kevin found out much later that Mako had been quite ill while working 8 hour days in the studio, without indicating to anyone that anything was wrong. It was a blow to lose him before finalizing the film, but in the end, they used only his takes from his first session rather than use a voice double. According to Kevin: "he gave that perfect father figure presence to the movie, and we didn't need alternate takes".
As for moving forward with a sequel, Munroe is contracted for a sequel script with an option to direct, but all that has been discussed so far are hypothetical storylines, wanting to give more time to Donatello & Michaelangelo since the other two turtle brothers got most of the spotlight this time around. Munroe is also interested in potentially bringing back the Shredder as a villain. Has also indicates a desire to dig back into the original comic series for the theoretical (though I'd call it almost certain) sequel.
He's looking to the widely-reported feature adaptation of Gatchaman next, but when asked what his dream project would be, he named two: an original comic creation of his own, Olympus Heights (Zeus comes to a small town in Indiana to subdue bad guys that keep popping up) and the DC Comics epic Kingdom Come. Kingdom Come, done in CG, would leapfrog the problems of using franchise attached characters like Superman and Batman, and wouldn't threaten the developing Wonder Woman property...which brings to my mind a similar route: how Marvel released their animated Ultimate Avengers movies and are separately developing live action standalone movies for Avengers characters like Iron Man.
After seeing the movie and talking with him, if anyone is going to be out there making CGI movies, Kevin Munroe treats adapted properties the right way. I trust him exponentially more than the mass-market, unfunny, condescending glut that's been out the for a few years. Supporting TMNT is good for CGI movies, supporting the other crap means letting the terrorists win.