So I have been out of the loop for awhile, but apparently Meltan/Melmetal are getting their promos for January, so that covers the last missing Pokémon from TCG in Japan. Blacephalon is still missing a regular card, and Hapu and Molayne didn't get Full Art cards, but obviously they will be covered soon.
This means our structure looks something like this:
*Before May: Titles of the 2019 games.
*April 2019: New Pokémon/form for 2019 games for the movie.
*May 2019: First trailer possibly with Detective Pikachu.
*June-November 2019: Pre-release period.
Edit: Since Meltan and Melmetal are going to appear in the TV series proper, this means next movie's new Pokémon will be more than likely a new Mythical, as Mewtwo has two Megas and Mew has an unique Z-move.
Edit 2: Since it doesn't seem like we are getting a Pokémon Direct for January 2019, this means the next likely date is the Pokémon Day, or February 26th.