In the 1980's during the Japanese Economic Miracle, some thought that Japan would become the second world superpower, and some hopefuls even thought that Japan would surpass the US in terms of economy. While that might be a stretch, I have a challenge. Prevent the 1991 Japanese Economic Crash and have the Japanese Economy keep growing, thus tremendously increasing Japan's influence and making them the firm #2 most powerful country in the world after the United States.
The continued growth would require increased worker`s productivity or exponential growth of workforce.
Increase of productivity has become difficult as infrastructure companies (including transportation) has grown increasingly lucrative and monopolistic (hence ineffective), stifling the growth. Forced breaking of Japan Railways in 1987 was too little and too late. Also archaic banking system which survived to today in form of numerous "zombie banks" did not help.
Regarding growth of workforce, this was already problematic by 1991, as birth rates has already started to plummet since 1974, thanks mostly to stifling regulations related to housing. Many housings built before 1970 were actually made illegal by new safety-based rules. Actually Japanese stagnation is well coincident with the first drops in admissions to career positions. Japanese politics did again too little to import skilled workers, a debate which linger to 2018 at least.
Therefore, for longer economic growth, i propose following:
1) Earlier dissolution of JR and Japan Post.
2) No government support for ailing banks
3) Prevention of the implementation of strict safety codes regarding housings and factory building. Will be a lot of excess deaths, but hundreds times more excess births too.
4) Less of nature conservation, more of forest plantations (to drive down construction material costs a bit)
5) Larger inheritance tax and land tax revised to speed turnover of land lots (to build higher-density cities, industrial zones and reduce road transportation overhead). Also, larger tax on parking lots based on footprint area, promoting high-density city layouts.
6) Promotion of wide-scale immigration, allowing newcomers to compete in all sectors and keep wages low.
7) More reliance on nuclear energy, combined with more sensible power plant site selection (this is related to (5)). Growing economy does mean growing power consumption, while oil-fired plants generally have much more air pollution problems.