I really appreciate all your comments, reminders and suggestions. I am totally aware of how difficult in design of a true product, e.g. if it is playable, if it is working well to tell a story, and appealing enough to both kids and AFOLs, or any conflict if it is related to a licensed franchise. After a rethink on your questions, I realized this project is better to stay as a MOC rather than a CUUSOO.
I am an AFOL and I love LEGO. That's all. To be honest, this is almost the first time for me to experience CUUSOO. I just followed the instructions from CUUSOO webpage to post, to propagandize...I never expected I can get or collect anything from such a little MOC. Maybe I shouldn't use CUUSOO as the platform for a purpose to merely share my creation. It is too much beyond my original thought.
I think I'd better avoid using any TMNT franchise and just leave it as a general sewer vignette, because it doesn't tie into any existing scene or lore from TMNT. I just made it totally out of my own imagination. What do you say, any further suggestion? Thanks.