A higher TFR could help. More people means more products. If Japan can manage a TFR of 1.9 for the last 40 years we would have an increase in the number of Japanese people.
More relaxed immigration could perhaps help as well.
Some Japanese companies could also benefit from better management. Just looking at how many mistakes Nintendo makes but gets away with because they have the IPs so they can still carve out a niche when they could dominate the market completely with better decisions and more aggression over the last 25 years.
Still, Japanese culture has been wanked really hard OTL. Maybe some of these fixes would really just kill the goose that laid the golden egg. Maybe a Japan with more immigrants, more English speakers, less traditional Japanese values, less conservative companies, more women in the workforce, etc... would undermine things that made them unique and popular in the first place? I don’t know the answer to that sort of thing, but maybe tinkering with Japan would only undermine their tremendous success?