OK.
Changes that would result from this :
0. Japan not being an American ally (like the Japanese rump state was IOTL) would prevent (or at least reduce) American expansion into the Pacific. No american Samoa, Bismarck Islands or Hawai'i here! Possibly no American Congo, either.
1. Japan would, in this scenario, retain the southern Kurils, Ezo/Yezo [OOC:Hokkaido}, Kyushu , and the Izu Islands and not have to retake them later.
2. It will also probably be richer than IOTL
3. Japan would probably protectorate Luquiu instead of Korea ITTL (it's better positioned to do so)
4. Japan would be better positioned to attack the Spanish in 1898 than Korea was, and the USA will not attempt to annex them ITTL, so the Philippine War would go differently. Taiwan would end up as part of Japan (not Korea). I think Luzon probably ends up as a Japanese colony or protectorate and the Visayas could go to either Japan or Germany. Both Luzon and the Visayas were occupied by America IOTL. Preussen Ost-Mindanau and the takeover of Sabah, Palawan and western Mindanao by Sarawak still probably happen.
5. Spanish Micronesia would probably become Japanese rather than being split between the USA (which took Guam), Japan (which took the Izus, Bonins, Ogasawaras and northern Marianas) and Prussia (which took the rest)
6. Japan would be politically more right-wing than IOTL.
7. Japan would also be way less multicultural than IOTL. After WW1, Japan found itself with not insignificant minorities of Han Chinese (on Kyushu) and Poles, Green Ukrainians, caucasian Muslims, Jews and Aynu (on Ezo), most of whom had fought against the Russians and Koreans and were therefore considered friendly. (on Ezo/Yezo, Japanese were only a plurality, in fact, even after the deportation of most of the Russian population of the island). ITTL, the only minority in Japan would be the Aynu, who have a low population and mostly live in the most remote areas of the most useless Japanese island.
8. The Russian Empire collapsing is basically inevitable with a PoD after the Napoleonic Wars, and the Soviets (or an equivalent group) probably still win from the resulting collapse. However, there would be changes in Siberia. Japan would fight against the Soviets and Green Ukraine along with the international intervention rather than being co-belligerent with the Soviets (and the second country to recognise them and the first to recognise Green Ukraine), likely gaining Sakhalin and the northern Kurils and possibly gaining Kamchatka as well. It is possible that this would result in everything east of Yakutia and Buryatia remaining under tsarist rule.
9. These, of course, assumes that Japan modernises to a similar extent to Korea IOTL. I have read a decent timeline where Japan basically becomes an Asian iMerina [OOC : Madagascar], managing to remain mostly independent in foreign policy and keep its territorial integrity roughly intact, while still having some areas (Ezo/Yezo, the southern Kuril islands, Izu, the Osumi islands, Hizen, Iki, Tsushima) turned into foreign concessions.
@HIM Dogson, Considering that only about 30 samurai fought in the American civil war, the lack of them would probably change literally nothing. Please do not confuse Kurosawa's movies with reality.