This thread primarily concerns whether or not Venus de Milo specifically should be rebooted.
But on the topic of more new turtles:
As long as any newly introduced turtle characters are well-written in their own right, I'm not about to care how many different mutant turtles grace a story.
And in a place like Mutant Town, there's always the chance of incidental supporting characters or random background characters who are neither ninja nor "one of the Turtles," but just happen to be mutant turtles. Because whether the next Mutant Towner happens to be a turtle or not is no longer all that remarkable, which is precisely why Raph and Jenny can now walk around in public without issue. For example, when Mona Lisa met Jennika for the first time, Mona at first wondered if Jenny was that turtle ninja vigilante (Raph) that Mutant Towners had been seeing around, but Mona otherwise found nothing remarkable about Jenny being a turtle, since it was no more remarkable than Mona being a lizard or Alopex being an arctic fox or Sally Pride being a lioness or Diamond being a porcupine or Sheena being a pig.
This is now a scenario more like After the Bomb (also the name of Sheena's band), where mutant characters can outnumber human characters, and you shouldn't be surprised by all the different species you see. So when you said "no more new turtles," I cringed. If you were to say such a thing in the context of this story, it begins to sound just like saying "no more new raccoons" or "no more new lizards" or "no more new porcupines" or "no more new Mutant Towners." With the story inviting its readers to sympathize with the plight of up to thousands of Mutant Towners with such diverse appearances, as well as this particular plot's blatant allegorical similarity to the real world's unhappy history of involuntary urban ghettos and urban separation walls, I shouldn't have to explain why "no more new XYZ" starts sounding uncomfortable in context. You may have meant "no more new turtles" in the context of the overall series or the TMNT mythos as a whole, but considering that all these characters were forced to live in a glorified open-air prison because of what they are rather than who they are, your words sounded...especially wrong.
You see, Sophie's storytelling here has proven extraordinarily effective and immersive, holding its own just as well as any other TMNT writer's material I've ever encountered. When I saw the albino turtle for the first time, I wasn't thinking of how she might affect the balance of turtle characters in the story—I saw a homeless child and cared about what would happen to her. At the same time, Jenny was meeting with Al at Alopex's shelter, which Al explained is for the all too many Mutant Towners who are homeless, hungry, cold, or still struggling to cope psychologically with their change. Al mentioned seeing a lot of unaccompanied homeless children in Mutant Town, and explained that the common wisdom was that many of these children have still-human parents living outside Mutant Town who promptly disowned and abandoned them to their fates after the bomb. (And it was similar for many homeless adults, who had been fired from jobs, evicted from homes, kicked out of schools, abandoned by loved ones, etc.) Something similar has happened before now and then in the mythos, and since parents forcing their kids into homelessness is something that still actually happens in the real world for other, nonfictional reasons, storylines like these can still resonate with readers. And considering that another IDW regular turtle, Raphael, also started his story as a homeless youth after mutating and being separated from his family, I can hardly think of a more appropriate way to introduce this albino turtle into the story 100 issues later.
Who knows? Sophie isn't telling anyone this character's name yet, but maybe she's the new version of Shadow.