Just to put things in perspective
Here are how the last 8 years of main series games were revealed
Black & White - Official site title, CoroCoro first reveal
Black 2 & White 2 - Pokémon Smash TV show
X & Y - Pokémon Direct
Pokémon Bank - Pokémon Direct
Omega Ruby & Alpha Sapphire - Random video
Red, Green, Blue, Yellow Virtual Console - Nintendo Direct
Sun & Moon - Pokémon Direct
Ultra Sun & Ultra Moon - Pokémon Direct
Gold & Silver Virtual Console - Pokémon Direct
Crystal Virtual Console - Official site
Here's how the last 8 years of spin-off games were revealed, going backwards from the most recent
PokéLand - Official site(Mobile)
Pokémon: Magikarp Jump - Official site (Mobile)
Pokkén Tournament DX - Pokémon Direct
Pokémon Duel - Official site(Mobile)
Pokémon GO - Press conference(Mobile)
Detective Pikachu ~Birth of a New Duo~ - Official site
Pokkén Tournament - Pokémon World Championships
Pokémon Picross - Nintendo Direct
Pokémon Super Mystery Dungeon - Title reveal through PR, first official showcase a week later in Nintendo Direct
Pokémon Rumble World - Nintendo Direct
Pokémon Shuffle - Nintendo Direct
The Band of Thieves & 1000 Pokémon - CoroCoro
Pokémon Art Academy - Nintendo Japan Video
Pokémon Battle Trozei - Nintendo Direct
Pokémon Mystery Dungeon - Gates to Infinity - CoroCoro
Pokémon Rumble U - CoroCoro
Pokémon Conquest - Jump Festa
PokéPark 2: Wonders Beyond - CoroCoro
Pokémon Rumble Blast - Nintendo Video
Learn With Pokémon: Typing Adventure - Official site
So Direct reveals are typically for spin-offs, but not exclusive. As most have been mobile lately (due to Ishihara not believing in Switch), it's hard to gauge. If a Pokémon game is revealed in the Direct, chances are it'd be a spinoff and chances are it'll be Detective Pikachu :P