Say the JRP suddenly found itself in charge of the Japanese government. What would they change?
Everything outside of foreign policy will become more decentralized. JRP believes in "prefecture rights" so if JRP were to become the ruling party the prefecture governors and prefecture assemblies will become much more powerful and a good part of the central government budget will be devolved down to them. Assuming JRP came to power sweeping Western Japan plus various urban centers in the East which is their long term electoral plan, then I can also see JRP moving to de facto divide Japan up into 2 or 3 zones with Osaka being the de facto capital of West Japan and Tokyo being de facto capital of East Japan. I can even see a Middle Japan zone where Nagoya becomes the de facto capital. Deep down JRP does not like the central role Tokyo plays in economic and political life and anything to shift power away from Tokyo will be on JRP's agenda.
A more aggressive foreign policy will be something JRP will continue to centralize and push for with higher levels of funding for the military.
This assumes that a JRP that wins power will still care about regionalism. Spoiler alert: They won't. They only care about regionalism now because they're an Osaka-based party who wants to have more power to play in their own sandbox. If they were winning nationally, regionalism would no longer be appealing to them because it would just preserve the power of local LDP and/or CDP administrations in prefectures JRP didn't control.
The JRP's governance would be literally indistinguishable from the LDP's, even in terms of military issues IMO, although that is less certain.
