In a recent interview, Game Freak director Junichi Masuda confirmed that Pokémon Let’s Go Pikachu and Let’s Go Eevee aren’t spin-offs. He also stated that future games could also connect to Pokémon GO. Masuda’s full comments are as follows:
“These games aren’t spin-offs. These are core Pokemon titles. As for whether we would make other games that could connect to Pokemon GO, that will depend on how the games are received. If we hear a lot of people saying that they enjoyed being able to bring Pokemon to these games from Pokemon GO, then we’ll think about maybe having future titles also be able to connect to it.”
Source: Nintendo Everything
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