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Theory time.
2016: Pokemon Stars development starts
2017 (mid): after Zelda BOTW success, ambition grows. Start collaboration with Monolith after XC2 is finalized. At the same time, the mobile team starts developing Let's Go, a project designed to be on the market quickly.
2018: Pokemon Let's Go released. Monolith and the main Pokemon team still working on the "BOTW of Pokemon".
2019-January: first trailer. Also a mobile port of Pokemon Let's Go announced.
2019, but likely delayed into 2020: release of the "BOTW of Pokemon"