The next Japanese election is at least 2 years away, but there have been some developments that may be of interest.
Japan's Democratic Party has chosen a new leader, Renho Murata. Renho is notable in that not only is she a woman leading a major party in Japan, but she is only half Japanese, as her father was from Taiwan. Japan has had a female leader of the opposition before, but it is extraordinarily rare for anyone who an immigrant, or a child of an immigrant, to rise in Japanese politics. So rare that when it does happen, it becomes famous for simply happening.
Renho will lead the Democratic Party, a successor to the Democratic Party (into which the Liberal Party merged) which was itself is a successor to the Democratic Party. The party leans slightly to the left on social issues. The main opponent of the Democratic Party is the Liberal-Democratic Party, created in 1954 by a merger of the Democratic Party, and the Liberal Party; which itself was created from a merger of the Democratic Liberal Party with the Liberal Party.
The LDP is the current government lead by Shinzo Abe, and leans to the right. Perhaps the biggest difference between the parties is they are shifting, slowly but surely, towards the directions of the 'new' right and left dichotomy. While the LDP backs free trade, some of their policies can be compared to that of Donald Trump, even if those commonalities are limited in number. The DP meanwhile, as exemplified by electing a half-taiwanese female leader, is much more 'global' in it's view, and behaves in a way more common to parties in the western world, while the LDP has much stronger tied to "tradition" in the traditional Japanese manner. Polls indicate that the LDP has a strong lead going into the next election.
However, polls in Japan are difficult to read. If you read polls going back to 1998 you'll notice a trend in most; they are woefully under-estimate opposition support.
A poll in June of 2000, the time of an election, indicated LDP support at 28.1%, which may seem low. This is because half of all respondents tell pollsters they back "no party" at this time. As such we can roughly double our numbers to correct for this. The LDP would thus be sitting on 56% of the vote, compared to a rough 16% for the Democrats. The real election, however, saw the LDP take 41% in the constituencies, and the DPJ take 28%. It gets even worse as in the Proportional seats, the LDP took 28% to the DPJ's 25%. November 2003 hit closer to the mark, but still under-estimated opposition support. This repeats again in September 2005. In August 2009, the opposition actually won the election, and it was the first time polls were accurate in any meaningful way. In December of 2012 the LDP won government back, and the polls, again, were useful... but only for the top two parties. Polls woefully under-estimated support for a 3rd party, and were still far off the mark in terms of PR support. The old trend returns in december 2014, with polls suggesting the DPJ would take 1/4th the support the LDP did, when they actually took half.
This is when I remind people that Political Science, despite the name is, an art, not a science. As such I have no hard number I can give you, only to tell you that you must increase the support for the opposition parties if you want to get any sort of accurate election prediction from Japanese polls.
This brings me to recent polls that show the LDP on 40%, and the DP on 8%, with 40% of voters providing no party support. Given the history shown, my current election prediction would be as follows:
285 - LDP
100 - DP
40 - JCP
35 - Komeito
15 - Others
Komeito sits in coalition with the LDP, and the JCP for those who don't know is Japan's Communist Party.
This would be a loss of 5 seats for the LDP compared to what they currently hold, and a gain of 4 for the DP, while Komeito would hold steady. The JCP would be the big winner, nearly doubling their seat count and capturing their largest number of members ever. This is due to the fact that no other opposition party exists for those who dislike the DP to vote for. While there are small parties, they are polling so poorly that even a combined 15 seats may be optimistic. The JCP itself is not much more radical than the Czech Communists; though comparing two parties a continent away is always difficult.
Should the above come to pass it would mark a possible return to 'stability' in Japanese politics. It may be that any future DP governments will lack the ability to obtain a majority, in which case they would need to rely on the JCP for support. If so this would create an effective "Two Coalition" system where voters are given a clear choice on which coalition will govern them.
How this all plays out remains to be seen.