It won't be Gen 9. That'd put Gen 8 at the shortest generation of all time and give the game a 2 year development cycle, which just doesn't happen
Just because it's the 25th doesn't mean there will be a shift or any big huge reveals. Remember that. It falls right-bang in the middle of Gen 8
At best we'll get
A new Gen 8 game, perhaps remakes or hopefully something new in an old region. This would likely also bring the remaining "culled" Pokémon into the Gen 8 existence (We'll have ~230 missing after Crown Tundra is out)
Spin-offs: New Pokémon Snap, Pokémon UNITE
Anime as always, with a new movie
TCG Promotions in the west
A crapload of merch
Tie ins in Masters and GO, possibly involving Party Hat Pikachu in GO
Also don't forget that games aren't in development when the last one finishes. For example Let's Go didn't start when USUM finished and SWSH started when Let's Go finished. SWSH development started in 2016 with USUM and Let's Go being done at the same time by other teams.
Things are done in unison by different teams and people working on different projects as they're needed: i.e. an art designer wouldn't be needed after a point in development for one game so can shift over to another project while the original project is still in development