Please let me answer straightly as I am not so young as to hesitate to say something.
Before talking about Japan's future defense, it would be necessary to think back what happened in the history again.
In the first place, isn't this a major premise that Japanese original military power had been disbanded in 1945, not voluntarily, but by the US so that Japan had no choice but relied on the US military power in the Far East?
Traditional Japanese conservatives, mainly represented by LDP, have been wishing to have Japan's independent military power which is never controlled by foreign powers for the past 77 years. I think that this will be same for any countries because this is a matter of national pride but I do not necessarily think that this will mean immediate conflicts with neighboring countries when the war with them had ended in 1945. What the US currently says "common enemies" with Japan is basically the enemies for the US. Please do not forget that Japan is a member of Asia before a partner of the security treaty with the US.
Sadly, the presence of the US military power in the Far East has been posing the danger of military conflict between the US and Japanese neighboring countries, which was not necessarily a good policy for Japan's security and is completely counterproductive for Japan's economy too. If the US troops may withdraw from Japanese territory in the future, naturally, Japan will have no choice but to increase its defense budget to make up for the lack of defense capability but, remember, most of that budget can be used for the domestic industries and new employment more than ever, which is economically favorable certainly. Mitsubishi Heavy Industries, for example, will not have to suffer from order shortage to the Ministry of Defense anymore as it has been on the edge of bankrupcy for many years.
Since Japan did not have chances enough to grow such a Military-Civilian Complex as other countries did, it will be hard for Japan to turn into an arms exporter so soon but, of course, Japan will be equipped with the nuclear weapons. This is absolutely necessary as a deterrent. However, ultimately, Japan will aim for a neutral position in the region based on its own historical experiences in my opinion.