Just a topic aimed at technical aspects, i.e. the kind of toys the boys like to talk about, ships, aircraft and other military technology.

Trying to identify specific technical decisions that could have been made to increase Japan's military effectiveness.
One of these is the Ki-43, historically it first flew in late 1938, but was only put in production in 1941 after having to be redesigned. But if the IJA chaps were thinking better and looking around more, maybe seeing all those monoplanes with retractable gear in action in Europe would have compelled them to relax a bit that agility requirement (i bet even without the butterfly flaps it would still have been more agile that anything US or british, maybe "just" equal to the Zero but still good enough) and get Ki-43 in service in 1940. Would also have been good to adopt a reflex sight like the Zero. This would have gotten them perhaps 6 or more sentais of Ki-43 in December 1941, as opposed to just 2. Perhaps by this time they would have managed to at least partially solve the fragility problems of the Ki-43-I.
Historically they have build the hopelessly perimated Ki-27 until late 1942! Imo i would trade 3 Ki-27 for 2 Ki-43 any day.
Introducing the Ki-43-II in service earlier than historical would have also been good, i don't know why it took so long as the Ha-115 engine is bassically the Sakae-21. It could have entered production at the end of 1941 or early 1942 and available in number in summer 1942, about same timescale as Zero-32.
As for the IJN, well Shiden is a good example of a lost opportunity. Historically they gave top priority to the N1K1, a flawed concept, but have they not found out about the P-38 or F4U by 1941? If the IJN would have recognized the potential of a land plane version of the Kyofu, instead of being just a private venture the N1K1-J should have received full support, leaving N1K1 as secondary. Concentrating on actually making a better landplane out of the N1K1-J, i.e. low wing, perhaps a more aerodynamic rear fuselage (a sort of N1K2-J but appearing in 1942), maybe using a Kasei-20 series engine initially with Homare to come later, would have created a usable fighter by mid 1943. I'm not sure if cancelling the J2M would be a good idea or not (certainly more of the J2M5 versions would have been useful), as it also uses the Kasei engine. Perhaps it's better to built as many Shiden and Raiden as possible while reducing the now obsolete Zero numbers as much as possible (Nakajima built the A6M2 until 1944 if i'm not mistaken!).
There are others i can think and i can detail later like the Ki-100, A6M8, A7M, the P1Y, not to mention warships and so on.