Although it hasn't been announced yet, it seems like 'bells' aren't the only thing we'll be using to buy things with in the upcoming Animal Crossing: New Horizons. Nintendo has updated their homepage for the game, and it now clearly states that the title will include "In-Game Purchases".
Nintendo has previously said it wants to do more DLC for its games, so this should not come as a surprise - the Animal Crossing games are basically built on the back of cosmetic items, and this makes them very suitable for microtransactions. That said, it could also mean content expansions or even something completely different.
While this news likely will worry some, it is worth taking into consideration that Nintendo's DLC so far has, generally speaking, been of decent quality and well worth the money.