Okay, that helps quite a bit.
I doubt I'll download MUGEN, though; I've got too much stuff on my comp already. Perhaps someday, though...
Anyhoo, here's a few ideas:
1. Custom Pairing Battle Intros: I'm sure there are loads of amusing comments that could be used for a pairing between Raph and Mike, as the two of them have always made fun of each other. To choose one off the top of my head would be impossible, as one could easily fill a library with all the cracks they've had at each other's expense. I also think it'd be funny if characters acted surprised in the event of a "Mirror match", perhaps with a "!?" over each character's head as he realized he was fighting an exact copy of himself. Perhaps a "What the shell?!" voice clip could add to such a situation.
2. Taunts: Oooh, man... once again, so much to choose from... here's what I'm thinking...
Leo: I can imagine Leo thrusting a sword forward and pointing at his foe as if he were leading a charge, or perhaps as if to say, "You're MINE!" Whether a voice clip is really needed for this, I'm not really sure... but I think there's a buhmillion tough guy dramatic clips of him saying something about honor or victory or family or something else along those lines.
Don: Perhaps that clip of Donatello from the first 2K3 game saying "Let's play stickball... I have the stick, you be the ball!" would be less annoying as a taunt, rather than spoken with every other attack like in the aforementioned game.
Raph: If you're going with the 2K3, Wolverine-ish Raph rather than the OT, Chandler-esque Raph (The OT one is my favorite, but for the sake of a fighting game that'll need a lot of grunts and "Hi-ya!"'s, I can understand the choice you made easily), perhaps his rage is best summed up as a simple roar... But alternately, I would recommend a clip of him making a comment that he'd made to Hun in the first season of the 2K3 series, one that is the epitome of taunt-itude: "Yo momma."
Mikey: I'd take the classic Turtles in Time "Pizza Time!" sprite, but replace the audio with Mike saying "Pizza Time!" from the first episode of the OT series.
(BTW, I doubt I'll download MUGEN in the near future; I've got too much stuff on my comp already, and I've had problems with it as well, so I've been extra-cautious and conservative about what I download/put/keep on this thing. Maybe someday, though... Until then, I'm happy to help you make a game that lots of other MUGEN players and Turtle fans will enjoy.
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3. Ooh, coolness! Which Shredder sprite will you be using? Original arcade Shredder? Turtles in Time Lightsaber-wielding Shredder? Or Turtles in Time for the SNES Super-Shredder? And what voices will you be using? Because I must confess, I can think of no greater taunt/opening than classic Shredder saying his infamous line: "Tonight I dine on Turtle soup!"
4. Excellent job, man... I didn't notice the voice tweaking! It's cool that you can edit voices as well as sprite graphics like that.
5. If I'm not mistaken, "QCF" stands for "Quick Center Forward", as in an attack that's quick and straight forward, as opposed to forward and angled, correct (My forte is mainly platformers and action games; not so much with fighters aside from Smash Bros. I confess to the sin of "button-mashing" there, which is another reason I don't have MUGEN.)? Hmm... let's see... As far as a QCF Kick for the ground, how about this: Leo plants his swords in the ground, but continues to hold the handles. In a gymnastic move, he uses the firmly-planted swords to push himself up and deliver a two-footed kick to his opponent. Not sure if that would be better as a "strong" rather than "quick" attack, but it may depend on how it looks when animated. That's all I have for now... hope that helps!