Oh, we'll get a Ki-84. Maybe not this patch, but surely within the next year. It is the last of the widely produced (relatively, anyway) planes in WW2 that is likely to see a lot of use in the MA. Granted, if we get a version with 4 20mm or 2 20mm and 2 30mm, it will see a lot more use than if we got the 2 13mm and 2 20mm one.
Even in the Ia form though, it has superior performance over the "Big 4" in certain areas (different areas for each of the "Big 4"), and enough firepower to see enough use to become one of the most used planes in the arena.
If it really turns as well as the Spit 9, you'll probably see a lot of N1K and Spit drivers switch to it, at least for a while. Whether they stay in it will depend on which version of the Ki 84 we get.