Well, I personally think the power with the greatest interest would be Russia. Russia was already expanding into Manchuria and had interests in Korea. From there, if Japan fails to modernize, Japan is the next logical step; solidifying Russia's foothold in the Northwest Pacific. Russia would probably target Hokkaido (most likely), or Chugoku (from Korea). Perhaps Kyushu as well. The US is next. Japan much resembles the Philippines as the gateway to China, but to North instead of South China. Britain is next, in order to extend its network in East Asia. France I don't see as being very likely at all.
I don't see China is very likely either; barring a Chinese Meiji Restoration that leads to them taking Japan's place. If that's the case, however, while Chinese imperialism may be a thing (concentrated mostly in the traditional Chinese tributary states, it has the significant advantage of being a traditional world power, with the largest population on Earth, and abundant natural resources). These things all work to cancel out the main incentives for Japanese imperialism. If China grabs slices of Japan, most likely is the Ryukyu Islands, as a traditional Chinese tributary state and a useful eastern base.