Wasn't there a person who's job title stated he was working on pokemon detective 2 and another yet to be released title that started the speculation?Just speculation.
Currently side game rumours are these:
Let's Go seuqel taking place in Johto and being a remake of Gold/Silver as the next step for that franchise. Note that the pic listed above shows Umbreon, Espeon, Ho-Oh and Lugia next to the Let's Go box art though.
Gen 5 remakes, since those are next in line for getting remakes. That or possibly being the next Legends game or both.
Detective Pikachu 2, since its been noted to be in development, some think it will release next year.