Even if it isn't technically a seaplane, I'm all for the addition of the PB4Y!
Seriously though, give me the Mavis.
Ever since I first saw it in Aces of the Pacific, I've wanted a game where I could fly it. Even more so, shoot and blow stuff up with it. While there may have been other games where you could fly it (I don't know of any personally), this one would be best at modeling her properly.
But for game play, the A6M2-N would be the easiest addition. As mentioned, wouldn't be a game breaker and since we already have a hanger at ports, very easy to add.