A closer look at Mario Kart 8's DLC expansion blurb has revealed that players will still be able to play the new courses, even if they don't actually own them or have a subscription to the Nintendo Switch Online + Expansion Pack.
Despite fears to the contrary: the information on Nintendo's official Mario Kart page reads:
"Courses from Wave 1 of the Mario Kart 8 Deluxe - Booster Course Pass can be played from 18th March locally or online in Friends and Rivals races, even if only one player owns the Booster Course Pass or has access to it as part of a Nintendo Switch Online + Expansion Pack membership.
"From March 22nd, courses from Wave 1 will also appear in the Random selection when playing Global or Regional races with anyone online".
The upcoming Booster Course pack was announced at the recent Nintendo Direct and will include tracks from past Mario Kart games (including Mario Kart Tour).
Eight courses will be released at a time over six waves, ending in 2023. That's a total of 48 new courses, doubling the number of current tracks.
The first wave will include Coconut Mall from Mario Kart Wii, Choco Mountain from Mario 64, and Tokyo Blur from Mario Kart Tour. For now, the tracks included in the following waves remain a mystery (although there are surely some guesses out there already).
This gradual 'wave' of releases will follow a similar format to Nintendo's additional releases for its Super Smash Bros. Ultimate DLC characters.