there's no WW2 in this timeline, so japan would presumably continue expanding outward into the pacific and continental asia (but i was planning to keep it so that china still holds the greater part of the continent; maybe japan would just be limited to the korean peninsula).
In your timeline, you need a very strong Emperor (not Hiro-Hito) who will be able to punish the officers of Japanese Army in Korea and Manchuria who started fights and invaded chinese territories without orders.
but what's the maximum extent of islands and other regions that japan could conquer and annex into their proper empire? australia and new zealand should still remain british or independent, and hawaii still remain american, but what else? could they perhaps expand to and annex parts of indonesia and even get to the indian ocean? or would they eventually try conquering hawaii and parts of the americas anyway? what would teh reactions of other powers in teh area be?
Try to conquer Hawai or any parts of the America will trigger a war with the USA and the outcome we know : total defeat of Japan and occupation of the country and probably atomic bombing
Try to conquer Philippines after the USA gave this country independance will also trigger a war against the USA.
Try to expand in Mongolia or in soviet zone of influence in northern China will trigger a war with the Soviet Union and a total defeat of Japan on the mainland and soviet conquest of Korea + atomic bomb on Japan when the SU will have one to pressure a peace treaty...
Try to conquer Oceania : Australia, New Zealand, british territories in central and southern Pacific, french territories in the Pacific : New Caledonia, French Polynesia will trigger war against France and UK. Even if one of the country is attacked, the second will join the fight by fear for it's own possessions... So probably a defeat and eventually a atomic bombing both UK and France. UK in 1952 with american help, France in 1960 without americain help...
Attacks on the french Indochina, british Malaya, Borneo, Singapour, Birma or India or any Commonwealth territory will trigger a war against France and UK.
If Japan wait for the decolonisation, these countries will probably decolonised pacifically and be a member of the Commonwealth or a french-sponsored organisation with the same goals and organisation...
Attacks on Dutch Indonesia will mean a conflict against the Dutch and probably the French and the British, because they will fear that the Japanese will attack their colonial possessions next...
So no, Japan will never expand in Asia, in America or in the Pacific without a war against the USA or a coalition from Europe. And expansion in China will trigger embargo from the western powers and the USA... And a kind of Pacific War because Japan is not able to continue war without oil and importations from these countries.
So a big Japan will have an Empire with the Ryukyu, Taiwan, Korea (if able to prevent rebellion and independance supported for exemple by a united China or the SU) and maybe a part or all of Manchuria but with a big ennemy in any kinf of united China... Maybe Japan will be able to keep some minors territories in coastal China : Hainan island, some Hong-Kong style territories...
Edit :
Best way to keep a asiatic empire, prevent any kind of unification of China. Very difficult with the KMT or the CCP supported by western allies or the SU...