Mario Kart Tour may offer some hints about the next DLC characters coming to Mario Kart 8 Deluxe through the Booster Course Pass.
8 DLC Characters We Want To See In Mario Kart 8 Deluxe
These Are The Mario Kart 8 DLC Packs We Want To See - Feature
My Predictions/Wishlist for a potential Character DLC to Mariokart
Mario Kart 8 adding Yoshi's Island track, Birdo in Booster Course
Mario Kart 8 Deluxe - DLC Courses, Characters, and Datamines
Mario Kart 8 Deluxe Getting Five More Characters in Future DLC
My MK8D DLC characters prediction. I think it's possible despite
Mario Kart 8 Deluxe update adds 5 more extra character slots
Waluigi - Wikipedia
Mario Kart 8 - Wikipedia
Mario Kart 8 Switch DLC with Birdo, Yoshi's Island gets a release
5 Characters Who Could Be Next in Mario Kart 8 Deluxe DLC
Mario Kart 8 on Switch DLC characters: Who's next? - Polygon