By default, any fifth Turtle would come in standing on a platform of "Token Female" because any potential "fifth turtle" would "have to be" female on account of the invisible and arbitrary "rules system" which governs these things nowadays. The conversation maybe doesn't END with, "We need a Ninja Turtle with a vagina", but you better believe that's what STARTS the conversation. And deciding that you have to make up a new character to check off a box is always getting off to a bad start. Because now you're not making story decisions, you're making political ones. You're not writing to the service of the story, you're writing to appease the gods of Wokeness and Inclusion.
Otherwise, "Should there be a Fifth Turtle" need not even be a conversation. No, there doesn't need to be one. It's that simple. "BUT they're all dude's though! That ain't right!" And NOW we're forced to have a conversation that isn't gonna end well and shouldn't even be a thing, because someone feels "left out".
You can maybe create a fifth Turtle who's also MORE than "just the token female". But "The Token Female" is still what she'd always be by default, because "She DOES Have To Be Female, though". She just MIGHT also be other things, too.
It's just not a very creative idea and it doesn't do much to justify itself. "Some little girls like TMNT, but are sad that they're all boys." I care SO MUCH what little girls think, man.
In a vacuum, those aren't great reasons to enact sweeping changes to a longstanding and established mythos. In fact, when you start writing to entice more female fans, and in turn you end up with stuff like "Mutant Town"... well, I start to forget why "gatekeeping" is such a bad thing.
It really just doesn't even need to come up. Like, to what point and what purpose? In-Story, I mean. I know the "Real Life" reasons and I completely don't care. IN-STORY, why should things be any more of a cluttered mess than they have to be? WHY even entertain the notion of five or more when writers can barely make four of them compelling? It just seems like a complete waste of time, done only to check a box.
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Why couldn't Splinter hypothetically be the "one in the middle" between Leo and Raph, if Leo and Raph were the only two who existed? "You both better learn to co-exist and figure sh*t out because I'm gonna be dead one day." Simple. He'd serve the same basic function as, say, Mikey or Don in trying to be the intermediary, except that in this case Leo or Raph would have a reason to listen, whereas with Mikey or Don saying it, they don't.
Don't get me wrong, I'm glad that there's four Turtles, but if we're being totally honest you can easily cut the roster in half by grafting a few traits onto This One and That One, and you can still do every story almost the exact same way. I say this because again, MOST of the time each of the four Turtles only exists in the story to play into their One Defined Trait; given that sad but inarguable fact, you technically DON'T even need four of them. You can make the Leader Guy ALSO be a Tech Wizard, and have the Angry One ALSO be juvenile and quippy, and guess what? You haven't actually changed a thing. In fact, I've always joked that if you combine all four Turtles singular traits into one character, you're left with One Whole Well-Rounded Character rather than four one-note cartoons.
Man, given how quickly each of the Core Four already fall into being written as archetypes, I wouldn't even wanna think about what personality trait would become #5's entire gimmick. Being unbearably Emo, maybe?
I mean c'mon, we've already got "Hyper-Serious", "Hyper-Angry", "Hyper-Goofy" and "Hyper-Brainy". What's LEFT for ol' #5? "Hi, I'm ____, I'm the Ninja Turtle who shops at Hot Topic and cuts myself."
I mean, you HAVE to give them One Singular Defining Trait before you can even get 'em off the ground, and all the GOOD ones are taken already!
I don't know. I feel like the more hoops one has to jump through in order to justify an idea.... maybe the idea simply isn't very strong. "Turtle #5" requires an entire circus tent full of hoops.