It would be nice, but I also think it's unlikely. The game requires a lot of tinkering to ensure that the roster is balanced whenever a new character comes out. Super Smash Bros. 4 had a much bigger budget yet they didn't get that many DLC characters. And people still complained about Corrin being unbalanced.
The announcement of DLC for this game makes me curious as to whether they might end up adding DLC to MK8D. Because frankly, I don't think there is a good reason for a separate Mario Kart 9 for the Switch, unless it really changes up the gameplay in some way (maybe the Double Dash rules with two racers and special items based on your racers), otherwise they should just add new content to MK8D through DLC is what I've always felt, because I want more Mario Kart, that is to say I want more race tracks, and I suppose racers and vehicles, but frankly I think they've nailed the gameplay down pretty solidly, so I don't see a reason to create an entirely new and different Mario Kart 9.