To keep it simple, and get the most bang for the Buck, and to free up the present restrictions often placed on Allied Aircraft:
Ki 61 Ib (MG 151's), easly added since it is a varient. (New Gunie)
A6M3 (Type 99 MK II's), easly added since it is a varient. (Slot. New gunie)
Ki 84, a Must for all areas in 44/45 plane set's.
Juddy, much better atack plane than the Val becomes available in late 42.
I would avoid the Ki 43, it helps not all in adreasing present set up issues, The J2M is a nice plane but it is available at almost the same time as the George and it is not as good a plane, the Japanese canceled it in favore of concentrating on the George. The Ki 100 is a nice plane but not as good a preformer as the Ki 84 and not even as good as the late model Ki 61's. Non of these planes (J2M,Ki 100,Ki 43) help adress balance issues when set aganst the present allied plane set.
If the Ki 61-Ib were added and the A6M3 big early war balance issue would be adreased, espichaly if coupled with a Juddy adation, and a Ki 45.
If you want to adress a ealr war Pac Bomber issue and dispence with the Boston Peggy combo, a G4M (Betty)/ Beaufort combo would be best to take their place.
For Late war the Ki 84 is defentaly Key, the Ki 44 is nice but depending on what model and when it is used the Ki 61-Ib is better than it early and the Ki 84 later.
Adding more competative atack platforms is defenatly a good idea, the Francis,Jill, Grace, Randy are all exceelnt planes, espichaly the Grace and the Randy, these would certainly help in the ordance gap issue.
But I think to keep our wish list withen the relm of the posable in the short term:
Ki 84, Ki-61-Ib,A6M3, Juddy
Just these 4 planes would do a lot and requier only two new aircraft.