Man, does this bring back memories. Kudos to anybody who knows Splinter and Shredder's real names... And yes, I still remember them. I need a life. Badly.
But back to the game, I hope it's not a Street Fighter clone. The GC controller just isn't suited to that style of gaming. If I were designing the game, I would make it cel-shaded, obviously. It would be third person, with gameplay similar to Devil May Cry - high on visual flair, emphasis on action, but without all the gothic imagery. It should try and capture the cartoon's action sequences. Probably have to limit it to single player, and cut out a lot of the explicit violence to keep it a Teen rating.
The missions would be laid out in a storyline sequence. Kill the Foot Clan in this neighborhood, rescue April from Mousers, etc. All of it culminating with a boss battle against Shredder at the end.
Konami is sitting on a gold mine of nostalgia in TMNT. If they can capture both the younger market who is watching the new cartoons and the older demographic who remembers with fondness Leonardo, Donatello, Michaelangelo, and Raphael... They could make millions.