Actually, what we really need is a Japanese shift in its strategic thinking, preferably immediately after the Russo-Japanese War to give it time to percolate (change it much earlier and I expect that you may as well just completely change the nature of the Meiji government).
What we need is for the Japanese to realize that the only way they can ever hope to win a war against opponents who massively outnumber and outgun them is to have better tech, better doctrine, and better political/economic strategy. A prospective POD for this would be the Japanese doing somewhat worse in the Russo-Japanese War; they still win, but the casualties are even worse and it becomes more immediately obvious to the Japanese government and public that the war was a much closer-run thing than had been hoped. In particular, we need the "banzai spirit will overcome machine guns" idea to be dead beyond any hope of resurrection.
In fact, we might posit that the Japanese Army is in enough trouble by the end of the war that they don't get any of Sakhalin; the resulting public outrage should suffice to cause a sea change in how the IJA and presumably IJN run themselves, and possibly a change in Japanese political and economic theory as well.
The first big change would be that the Japanese would attempt to integrate the Koreans into the Empire, rather than turn them into a subjugated slave race. Assume that there would probably have to be some pseudo-scientific ethnic bullshittery going on to allow for this to be sold to the public; as far as the government is concerned, the real reason for it is so that they can count on the Korean population as an economic asset and a manpower boost rather than a running sore in any future confrontation with the European powers. A possibility might be that the Japanese try to create some sort of Austro-Hungarian style Dual Monarchy, albeit with the Koreans having somewhat less power than the Hungarians did.
Meanwhile, the IJA and IJN are both being modernized post-war; the IJA in particular is being instructed to look into any potential force-multipliers that would allow them to break through defensive emplacements without having to build a ramp of corpses to climb over them first. The key here isn't necessarily any major change in the equipment of the IJA or IJN before the 1920s-30s; it's that the government would rationalize and massively expand investment in R&D in the hopes of achieving technological superiority to balance numerical and industrial inferiority. Indeed, the IJA and IJN are likely to be considerably smaller during this time period than they were in OTL; the money that would have been spent on military expansion is being allocated to the R&D programs and a more carefully directed industrial expansion.
If you want a really big change, have the Japanese decide to open their doors to "useful" foreigners. Money would be set aside to try to encourage leading researchers to come and lecture at Japanese universities, foreign investment with technology transfer would be encouraged akin to what the Chinese are presently doing in OTL, and in general Japan would be somewhat more accepting of foreigners; figure that the scale of Japanese opinion of gaijin gets tipped from "dangerous outsiders" to "exotic newcomers" in terms of the majority view of the outside world.
Assume that WWI proceeds as OTL; the Russians retaining Southern Sakhalin doesn't make up for the fact that they were forced to sign a peace treaty favoring the Japanese, and I don't think that the change in Japanese behavior will have large enough effects within 10 years to prevent the balloon from going up. Assume that the Japanese take somewhat closer note of the lessons of the war; in particular, the importance of aircraft, of tanks, of submarines, and of an adequate command structure in general. The disintegration of Germany due to blockade would also serve as a warning for this Japan of the dangers of dependence on imported resources in wartime; assume that on the one hand this provokes the Japanese to make more of an effort at self-sufficiency in critical supplies and on the other it makes clear the importance of having a merchant marine that is large enough to supply both military and immediate civilian needs and that is properly defended.
The biggest events of the 1920s for this Japan would be the seizure of Sakhalin during the RCW and whatever changes might be wrought by the Great Kanto Earthquake of 1923. While I think this Japan would be slightly less prone to utterly crazy military opportunism, I also think that taking Sakhalin would become a point of national pride here, and that the Japanese would give the Americans a big warm "fuck you" when they ask Tokyo to pull out after the RCW ends. Since the Bolshies probably don't have much of a naval presence in the Far East in the early 1920s, they don't really have much of a choice but to grin and bear it.
As for the earthquake, it'll hurt. It'll hurt badly. Maybe not quite as badly as in OTL, since TTL Japan will probably have tried to spread its industry and finance around the Empire a bit more, but it'll definitely have a major impact on the budgets for the next few years. I'm honestly not sure what this Japan would end up cutting.
What I'm ultimately hoping to lead up to is a Japan that is still likely to end up at war with the US in the 1940s (because even if TTL's Japanese won't jump China in the expectation of conquering it, they most certainly will intervene to prevent any one side of the Chinese Civil War from winning, and Manchuria just has too much potential industrial value for the Japanese not to find some excuse to grab it. Figure their territorial ambitions in the rest of China amount to Hainan, Shanghai, Guangzhou, Fuzhou, and Qingdao, to give them bases in strategic and economically useful regions, with any further expansion only being undertaken if the government deems it both feasible and useful), but that has developed a strong indigenous technological base and that is far more capable of integrating new territories into the Empire due to the removal of the more brutal attitudes of the Japanese. Assume that this Japan is probably about on par with the US technologically when the war begins, and somewhat ahead qualitatively due to the Japanese being obsessive about the quality of their troops and gear as compensation for their inevitably being outnumbered. The Japanese are also likely to have a slightly better understanding of American attitudes simply by virtue of being more open to foreign ideas, although the cultural gap is still going to be pretty wide.
If any of this sounds plausible, I welcome someone else to take a stab at making a TL out of it; I'm simply not likely to have the time in the near future.