From: St.Petersburg, Russia
So, why I started talked about AA artillery above^
General aircraft defense reinforcement proposal
1)Around 1934 Japanese government and IJA actually take note of the massive growth that Red Army Air Force undergoes at the moment, including mass production of heavy bombers that can theoretically reach Home Islands from Soviet territories in the Far East. Considering tense relations with USSR, this produces a scare, not unlike the scare caused by Luftwaffe buildup in Europe IRL. When asked what it can do to protect its country and the Emperor from the air threat, IJA sees that it only has medium flak guns, that might be less than effective against even relatively primitive heavy bombers, and its capability to design antiaircaft artillery of any sort is extremely limited as well. IJN, at the time, already has experience of successfully designing large anti-aircraft guns (Type 90 DP system), and good relationship with the Hotchkiss company, the latter ready to sell its 25/60 AAMG, specifically modified according to Japanese specs, to IJN. In the real life IJA was only able to develop a small-calibre flak gun by 1938 and another new medium/heavy AA gun of its own by 1943 (Army 88mm guns produced later being copies of a German design captured in China). In this alternative, IJA is less uncontrolled and due to the air scare, generals prefer to lose face by asking the Navy for their stuff, rather than lose more of it by being potentially unprepared for war. The design process for a family of 88mm high-ballistic guns for land and naval use is initiated under both services patronage. (As Japanese were able to design modern 76mm guns for Agano-class cruisers IRL before the war, I don't think this task is unfeasible). They are eventually accepted as 9cm/60 Type 97 guns (actual calibre 88mm). Their development also provides important experience with high-ballistic weapons, that will later be used in creation of the 120/55 DP gun. Production, though, remains slow, for both ground, and, particularly, naval versions, despite the break provided by longer period of peace. Only a few ground units are equipped with them in 1941-1942, and the newly built ships (auxilary combatants and valuable transports) begin to receive these guns only in 1943. They never come near fully replacing old 76mm guns.
At the same time, a wheeled version of Type 96 25/60 single is developed for Army use instead of the Oerlicon derivative they used historically. Effectiveness is not much different, but higher power makes them more useful as improvised AT weapons, and design standardization allows to produce more of them, providing at least a few of the best-equipped square divisions with attached small units of these guns, and creating more independent companies. Airfield units get flak increases earlier.
2)Meanwhile, by 1935 the Navy, seeing capabilities of D1A and B4Y, now entering service, and their projected successors, completely dispences with the idea of using 13.2 MGs for fleet air defense, except on small auxilaries, barges, and so on. The German model (Kriegsmarine was the first navy to adopt the combination of 20mm+37mm guns for close-range air protection, although their pre-war 37mm guns were really poor, not even fully automatic) becomes more influential, perhaps due to earlier contacts with German naval thinking, that, as postulated before, impacted IJN's sub doctrine. While the Navy is not about to go through the trials of changing its main automatic AA weapons again, particularly as Type 96 is found reliable and suited for mass production, the development of the single Type 96 mount is given high priority, and they start appearing on Japanese even before beginning of the war. IJN starts approaching Rheinmetall shortly before the war, interested in its 37mm Flak 36/37 guns. These, in their pre-war shapes, are found unsuitable for naval use, but acquired information (in combination with blueprints transported by subs during the war), helps Japan to start producing its own 37mm guns based on this design. Small numbers of them appear in 1943 in ground units and by second half of 1944 on ships. Production remains slow, fire control remains obsolete, etc, etc. But I guess this has a better chance of producing something during the war than trying to copy captured Bofors guns. And the fact that Flak 36/37 was quite lightweight should come useful when you lack good powered mounts.
3)AA rockets are not used.
4)120mm and 150mm Army guns are developed somewhat faster and deployed somewhat earlier due to cooperation with the Navy. More 120mm Type 3 guns are produced in 1944-45 in place of relatively obsolete and less powerful 120/45 10YT guns, some delivered to IJN's units in Home Islands.
5)Of course, this is all not going to help that much against determined and professional air attacks. For the fleet, even German standards of air defense will remain wholly unreachable due to limited production capabilities, and it is not like those were sufficient to protect shipping alongside German-owned coasts by 1944, even in conditions that very much helped the defending side. But hey, every little bit helps.