I wouldn't introduce the Ki-84 unless there was some way to balance it out, either with a small ammo load or some sort of performance inhibitor, and since I'm not a programmer, I can't make any suggestions about which way to go.
When I flew AW (and AW2, and AW3, and AW ME) in FR Pac, the Ki-84 pretty much was the #1 choice of ride because it combined the best parts of a fighter: speed and maneuverability with a flight model that was easy to fly. The Corsair and Hellcat were 2nd and 3rd, with the P-38 and Niki close behind. It wasn't unusual for 2 or 3 Kis to be going into a furball against 2 or 3 other Kis, with maybe a Niki or Corsair thrown into the mix. It got kind of old, but the Ki was just such a superplane that us mediocre pilots had to rely on it just to be competitive.
One of the things I like about AH is that its library of planes is pretty well set up so that no one type dominates the arena. I fly the Spit IX and P-51D (depending on what I want to do that night), and its nice to be able to fight against LAGGS, Niks, Hellcats, Corsairs, and FWs on any given flight. Sure, all of those planes may be better than the others in the library, but at least its a variety. In AW FR Euro it was mostly Spits, FWs, and Mustangs. In AW FR Pac it was Ki-s, Corsairs, and Hellcats.
What I'd really like to see is a P-40 of some sort. And a P-61.