Let's assume here the Soviets manage to occupy Hokkaido before Japan's surrender. What are they likely to do with it? What happens if they:
1. Set up a communist puppet state named PR of Japan/Ezo (most obvious)
2. Annex it as another SSR
Let's have some discussion, folks.
I feel like having Hokkaido under direct control under Moscow would have a lot of US generals shitting themselves and calling for a more hostile confrontation against the USSR. At least North Korea looks independent from Soviet control.
So, on what a Red Hokkaido would look like: I imagine there will be US naval training in the general region, possibly US-Japan coordinated amphibious attacks near the northern coasts of Honshu. Every time there's a diplomatic conflict the US may or may not send aircraft carriers near the island like the Taiwan Straits crises, as the island would be very dependent on the USSR for food supplies. There would also be Soviet naval bases on the island.
On domestic politics, there will be a perpetual call for becoming an SSR or ASSR, as did Tuva and Mongolia. It's a small population so reaching model country status would be quite swift; they may even serve as an "Asian" model for implementing Nordic economic systems, depending on how much independence they get from Moscow - and whether Moscow wants to play around with the idea and needs a "testing ground".
The Pacific may become more important to the Soviets since they have multiple coasts in the region, as well as more possible naval bases; Alaska would become all the more important to the US.
Just a portion of what I had in mind and most of what was clearly in words.