It better be a really big update.

The sad thing is that Nintendo has little financial incentive to add much to the game. New Horizons sold nearly 34 million copies as of June 30, 2021. And it's probably managed at least a million more since then. It will probably sell millions of copies from Thanksgiving/Black Friday until the end of 2021.

New Leaf got a massive update (but it was just about the only content update for the game) in 2016 to increase Amiibo sales and squeeze a few million more copies out of the game. New Leaf's current lifetime sales are about 12.93 million.

Brewster and The Roost could very well be the final content update for the game, excluding minor things like a few new seasonal items.