Seems like Nintendo president Furukawa will be pushing for more DLC to happen for games.
Source
We just saw this with Smash with them announcing DLC before the game even came out and I feel like this is how they plan on adding in the crossover items into Animal Crossing Switch and I can see them also adding other playable characters into Luigi's Mansion 3.
Investor meeting Q and A wrote:Titles like The Legend of Zelda: Breath of the Wild, Mario Kart 8 Deluxe, and Super Mario Odyssey are what we call “hardware drivers” in that consumers are very often interested in buying these titles when making a new hardware purchase. Titles released already in the previous fiscal years remain capable of driving hardware sales. The key is to figure out what makes these titles appealing, and how we can get consumers to understand that appeal. Going forward, we plan to incorporate add-on content and other factors that will keep these titles in the spotlight, so they will continue to sell alongside the hardware.
We take the value of software very seriously. What we want with our evergreen titles is to maintain their values as long as possible without lowering the prices.
Source
We just saw this with Smash with them announcing DLC before the game even came out and I feel like this is how they plan on adding in the crossover items into Animal Crossing Switch and I can see them also adding other playable characters into Luigi's Mansion 3.