I'm uncertain of the POD, but let's have the IJN be obsessed with damage control on their carriers, starting with Akagi. Perhaps Hōshō suffers a massive fuel vapour fire and magazine explosion in 1923.
ITTL, the IJN demands that CVs under construction and planned have the following design and doctrine:
1) All crew are firemen, all crew are trained in damage control. Damage control is drilled obsessively.
2) Underwater and vertical protection of avgas tanks and magazines is modeled, tested and perfected.
3) Fire suppression and safe removal of escaped fuel vapour is paramount in ship design.
4) Aircraft are armed and rearmed on deck, not in hangars.
Staying within OTL's IJN Naval treaty compliance, what else should we add to give the IJN the best chance of surviving torpedo and bomb attack? That said, we're not building armoured flight deck carriers, until Taiho anyway. To survive in the Pacific we can't have reduced air groups. If British-type cofferdam protection of avgas is followed, we'll need to keep the fuel capacity higher than in the RN to support the larger IJN CAG.
Damage control is based on the assumption that the carrier will be damaged, so we're not giving the IJN early radar or more larger CAPs through some new folding wing aircraft. Instead, let's get the carriers to survive as best they can when attacked by Allied submarines, torpedo and dive bombers.