Most likely some sort of parliamentary democracy as we saw IOTL. But with a heavier emphasis on US-style 'republicanism,' and a more limited role for the Emperor. A stronger civilian government is also likely, with the military being slightly less emphasized in governmental affairs than IOTL. That isn't to say though that one man's life instead of death would completely avert Japanese militarism and imperialism; Korea and Taiwan, as well as many minor islands near the Japanese Home Islands are still going to be conquered. However the Chinese campaigns, specifically in Manchuria, and the campaigns in South-East Asia are far less likely. In fact, depending how you play out the TL, you might see a strong but democratic Japan allied with the US and UK in WW1 (as per OTL), but with a more peaceful Co-Prosperity Sphere. Perhaps a series of bi-lateral trade agreements, eventually moving towards an anti-colonial, anti-western, anti-communist but anti-fascist political/economic forum. I wouldn't go as far as to say a bloc, but certainly Japan could enjoy mutual defense arrangements with the Philippines, French Indochina, etc.