I had the thought that perhaps it would be Pro Yakyuu with MLB teams, players, and stadiums. Then, after picking up RBI Baseball and RBI Baseball 2 on the NES, I looked for some information for them. As it turns out, RBI Baseball was called "Pro Yakyuu Family Stadium" on the Famicom in Japan. I thought that this was rather interesting, and something that would make for a very good game if this is indeed the case.
Of course, this is by no means official information, it's just speculation on my part. And I have no idea what the heck "Pro Yakyuu" means in Japanese, it could simply mean "baseball", and this wouldn't mean a thing. But it sure would make sense that they'd do something like that, since they already had a good engine to start with.
Food for thought.

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