This is likely gonna be a more traditional Gen 8, if I had to guess.
Source: Pokémon (Twitter)
UPDATE: Eurogamer just tweeted this from the press conference.
2019 core RPG (untitled) is set to be "in the style of X and Y and Sun and Moon". This should appease competitive players, with Let's Go! aimed at a broad audience, "a core RPG for everyone".
The Pokémon Conpany International said:
Finally, Mr. Masuda returned to the stage to give us a tease at the next major project in development. While Pokémon Quest and Pokémon: Let's Go, Pikachu! and Pokémon: Let's Go, Eevee! represent new and different play styles, he let fans know that a new Pokémon core RPG, developed by GAME FREAK, is planned for launch on Nintendo Switch in late 2019!