Do you plan on adding the turtle's adopted mutant fish sister from that one episode? I fell like she neverwould want to help avenge splinter since he raised her and cared for her deeply.

I don't. If I did put her in, the fact that she exists at all would contradict almost all the world building I've set up behind the scenes for how mutation works. She is a wrench in the works- thus she can never exist in Residuum.