Mario Kart Tour may offer some hints about the next DLC characters coming to Mario Kart 8 Deluxe through the Booster Course Pass.
Stealth on X: With 8 characters and 48 tracks added to an already
Mario Kart 8 on Switch DLC characters: Who's next? - Polygon
Mario Kart 8 on Switch DLC characters: Who's next? - Polygon
Mario Kart 8 Deluxe's new character select screen hints at more
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3 NEW CHARACTERS coming to Mario Kart 8 Deluxe!
Mario Kart 8 - Wikipedia