Unlike the IJN, the IJA was fond to readily adopt new fighters almost on yearly basis after ~1940. For good reasons - the Ki 43 was not well armed, nor fast (level flight nor in dive), nor it was known to be tough; Ki-44 have had high wing loading and still was not stellar with firepower; Ki-61 was plagued by engine problems, ditto for Ki 84. Ki-45 was too many cylinders for too slow speed and, initially, weak armament; other 2-engined types were too late.
So - how would've the IJA fighters developed with less issues? The engine types are the ones historically made in Japan, though you can alter the production of engines (in a timely manner, of course) in order to have enough of desired types made for the newly-fanged fighter force. Starting date is, say, 15th November of 1940 - the Ki 43 is in pipeline for production, and you can have some feedback from Germans. Note that Japanese economy can't support the gazzilion of engines & whole aircraft someone might intend to whip up here, so plan accordingly.
So - how would've the IJA fighters developed with less issues? The engine types are the ones historically made in Japan, though you can alter the production of engines (in a timely manner, of course) in order to have enough of desired types made for the newly-fanged fighter force. Starting date is, say, 15th November of 1940 - the Ki 43 is in pipeline for production, and you can have some feedback from Germans. Note that Japanese economy can't support the gazzilion of engines & whole aircraft someone might intend to whip up here, so plan accordingly.