The problem is, Heyman is the only mouth piece in the WWE worth a damn, everyone else sucks and doesn't even come close, and Nakamura would not go well with Heyman because Heyman is a natural heel and might overshadow Nakamura with his long-winded promos while Nakamura just stands there... awkwardly. However, to be at the top, Nakamura DOES need a mouth piece, because of media appearances and whatnot, it's a tough situation.
Maybe Daniel Bryan could advocate for Shinsuke? He's gotten pretty good on the microphone and the two men have history, the problem is, Bryan is still over himself and he and Nakamura would be competing for pops. I can't think of a GREAT, face mouth piece that wouldn't cramp Nakamura's style, and would talk just enough to get the message across.
I'd say maybe Nakamura doesn't need a mouth piece, and his limited English is enough, but again, to succeed in WWE you do, from communicating with McMahon to all the office people, the Godawful writers, and again, outside appearances.
But yeah, no f'n Funaki, first, because he hardly speaks English better and second, because he's a Goddamn joke of a scrub and comedy act.
Actually, with his style and charisma, maybe a face female mouth piece wouldn't be bad with Nakamura, like a face Alexa Bliss? I like her promo delivery and she's the type of heel fans don't hate, she's just labeled a heel for being mean but one could turn her on a dime.