5th Turtle?

[Cryomancer]

They already under-use tons of characters in most TMNT incarnations anyway, and the TMNT themselves have a tenancy to become fairly one-dimensional personalities as well.

tl:dr opinions on Venus:

I watched Next Mutation for the first time recently, and Venus was not as offensively awful as people make her out to be. If anything, she's one of the least obnoxious things in the show (when not trying to make jokes, which everyone fails at equally). The presence of Venus doesn't make the other turtles in NM decide to rapid-fire terrible catchphrases every time they start their car. Her defeating Shredder right away is pretty lame (although almost worth it to have homeless shredder later on), and the overall plot for the show is essentially she something brought to their doorstep, but otherwise, eh, I've seen worse characters. Whoever decided she should wear light blue is pretty dumb though, yellow or grey or something would have been a much more sane choice (pink probably being a bit too stereotypical). Probably the biggest problem she introduces is that the guys apparently want to bang her, and therefore now they start throwing around "we're so totally not related!" stuff which undermines the family theme much of TMNT shares. She did have a unique skill set and weapon at least. Defeating Shredder would have been better handled as something a bit more drawn out probably, or with her arriving just before they were close to defeating him anyway (or, you know, afterwards). She did bring 1) an extra body and, 2) a new form of "attack" to the battle however, so it's at least slightly tolerable.

Besides the best 5th turtle is Keno, he's a Surf Ninja.