According to the Wrestling Observer Newsletter, the COVID-19 restrictions in Japan are said to be easing up and venues are now allowed to be back at full capacity in most arenas, which will be a tremendous boost to NJPW.
NJPW would now be allowed to have up to 5,000 fans in Korakuen Hall, where they regularly run all of their events at max capacity. Buildings like the Tokyo Dome, Yokohama Arena and Budokan Hall are allowed to have over 5,000 fans but no more than 50% capacity (or over 10,000 fans).
This would mean the capacity for Wrestle Kingdom in January would be 10,000 in the building or less than that paid. If they decide to allow cheering or booing in smaller buildings, it would be back to 50% capacity. But they can allow that now if they decide to.
