The datamine indicates that there are no more new courses, which I suppose makes sense given how this game's new courses tend to coincide with Mario Kart 8 Deluxe: Booster Course Pass, meaning that for now, 4 more courses are slated for it (Rome Avanti, Madrid Drive, Piranha Plant Cove, Piranha Plant Pipeline) and it's doubtful that there would be more. I am more curious about the remaining racers, given how the obvious ones are out of the way (the last coloured Mii suit is the next one).
Even when the game stopped getting new content, I feel that the game will still cruise by because it's a far more popular game. Totally unlike Dr. Mario World where as soon as the last world got added, the game is already announced to be shut down. What they did with Dr. Mario World is way too abrupt so I hope they have the common sense to let this game cruise by a while longer, like I guess at least 1 year before the game starts closing.
Thank you for reading.