I don't recommend expecting too much from JCP.I wouldn't bet on the variety of "Democratic *insert other adjective*" parties. They are just as pro-US and pro-Taiwan separatism.
Best case scenario Japan's economy is devastated by a limited war over Taiwan and/or Diaoyu + climate change + population decline and people come to their senses, drop their largely irrational fears and completely elect the JCP (yes, unlike the "CCP", Japan's communist party actually is officially called "Japanese Communist Party") to power which then transforms Japan into a socialist state.
In a worst case scenario (for both China and Japan) I wouldn't put it beyond the LDP to try to transform Japan into a South American style junta-led military regime amidst a crisis, of course with US support. I'm skeptical whether the public and JSDF would actually go along with it- the concept of democracy is very entrenched in society despite historical denialism and anti-China sentiment in the media- but it is a possibility that shouldn't be ruled out as far as whatever China needs to plan for goes.
The junta scenario may seem outlandish but Aso Taro, who continues to hold a high ranking position, has literally said that the LDP should copy the Nazis in how they restored the German military and rearmed. The chaos that a conflict over Taiwan would cause would be the perfect situation to take advantage of.
There are many reasons. Let me list them briefly:
1、At present, the Japanese are convinced that they are on the "right side". Francis Fukuyama's attitude of regarding the western system as a religion has a lot to do with his Japanese ancestry.Even when we read those entertainment novels, we can often see that Japanese writers sometimes flaunt their country's "democracy".When communism is severely slandered, it is difficult to change people's minds under the omnipresent propaganda.Unfortunately, they always feel that they live in an ideal Utopia and feel that there is nothing to change.
2、The political enthusiasm of the people is not as high as expected, and the apathy of the Japanese is beyond imagination. In the last century, there were too many factional contradictions within the JCP, and the radical youth groups were easily driven by self moving emotions, which made the opportunity to gain influence wasted by a series of mistakes
3、The United States has very tight control. Some people satirize that the current situation in Japan is nothing more than the shogunate regime ruled by the United States.Since the United States has military base in the capital of Japan, it can allow the United States to take over the leadership of Japan by force at the first time.
4、JCP and the CPC already have great political differences,the two sides may not have the intention to cooperate.In order to cater to the voters, this party, which has been seriously marginalized in Japan, has become consistent with the LDP in many aspects.Now, the JCP is also criticizing China's "expansionism".
Don't expect anything from the Japanese.In consideration of their consistent attitude of admiring the strong,I'm afraid they won't change their minds until they are hit hard.yep, Japanese have only themselves to blame for allowing fascists to come to and stay in power since 1930's. Once this mistake has been corrected via denazification of the regime, I am sure the Japanese people will make the correct historical choices.