wispywood2344 , Anonymous and Gessler have separately brought up the issue of Japan considering the order of US Tomahawk cruise missiles while Japan perfects long range "Type 12 Imp" cruise missiles. Both missiles are capable of anti-ship and land attack missions. They are capable of being fired from land, aircraft, surface warship and (of most interest to this blog) from submarines.
In the spirit of its Peace Constitution Japan is remaining very subtle in developing "New Defence Program Guidelines" on future consideration of an "experimental" or prototype conventionally propelled missile submarine (SSG).
The construction of the submarine will start in fiscal 2024 with eventual testing perhaps out to 2036. If testing looks positive serial production of Japanese SSGs could commence in 2040 following the end of the Taigei-class submarine production period.
One missile launch method under consideration might be horizontal torpedo tubes - perhaps an increase of the 6 tubes in the Taigeis to 10 tubes in a new SSG class.
More radically and along the lines of Japan's South Korean "frenemy's" KSS-III SSG/SSBs Japan might consider vertical launch tubes. Vertical tubes can be larger, hence more versatile than typical 533mm (up to 650mm) horizontal tubes. Vertical tube versatility can permit larger types of hypersonic or ballistic missiles to be fitted.
Under Japan's Peace Constitution it does not want to spell out that its future SSG/Bs might one day be capable of firing hypersonic or ballistic missiles. Hypersonic or ballistic missiles could not only be Japan's touted "defensive" responses, but for pre-emptive strikes. Pre-emptive strikes are better done by "special" non-conventional warheads, but mention of all this is officially taboo under the Peace Constitution.
The main possible targets are not South Korea (even with SK's ballistic missiles) but the North Korean landmass. Conceivably future targets might also be vessels belonging to China (particularly in a Taiwan contingency, late 2020s onwards) and even Russian vessels.
Meanwhile, increasing Japanese use of LDUUVs and smart mines might also enhance Japan's ASW capabilities.
South Korean and Japanese conventionally propelled submarine capabilities are forging ahead, while Australia's (all in the SSN basket?) undersea future may hopefully be revealed in March 2023.