Who knows, it'll depend on if they decided to develop 29/31 for Nintendo (combined that plot could hold a huge showbuilding). and how big the Mario Kart showbuilding will be. That'll determine how many stages will go. But I feel like more are definitely going soon.
HOWEVER, I found an up to date satellite image of Japan's. It's REALLY wedged in there. This current site that is under construction (including the donkey kong dirt lot) is only 5.01 acres. So I do believe we will get the larger Super Nintendo World than Japan if SS28 is included.
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