I don't even see how people can think it'll get another MK. If you want new stuff, I don't see why any sane dev (that is not part of EA lol) would republish a game which would be exactly the same in terms of graphics and physics. Just update it or put some DLC. And if "bleh buttt Mayrro Kratt Wiii is da bestt Mrio Karr 8 is garbajje" shut up and return to your 2008 game.
But if its an entire separate game that can be connected with your copy of MK8, to share all the content of both games, it could be a great way of selling both 9 and making more sales on 8 Deluxe. As I said earlier:
I think we may see such a game, but if they do, I'd like to see it do something really special. Keep in mind that if the Switch got another Mario Kart game, it would be the first Nintendo system to receive 2 Mario Kart games built for the system (even if one is an enhanced port).
So with that in mind, if the Switch gets a Mario Kart 9, I'd like it to have a modern day "lock-on technology" similar to what Sonic & Knuckles could do with Sonic 2 or Sonic 3. If you have a digital copy of MK8 Deluxe, it'd be cool if Mario Kart 9 could integrate itself into that game and MK8 Deluxe could integrate into Mario Kart 9 to collectively make one large game known maybe as Mario Kart 9 Deluxe. I believe that 8 and 9 would need to have something uniquely different between the two titles to justify its existence beyond just being more DLC for MK8 Deluxe. So as an example, perhaps MK9 would be like the MK8 mechanics meets the Double Dash experience in that every kart would have 2 racers and you'd see the return of the special items based on your racers. On its own, MK9 would be only the Double Dash formula, but if you have MK8 Deluxe, the 2 games share all their features. So now you could play all the MK8 courses with the Double Dash formula, or you could now play the new MK9 courses in the traditional single racer mode of MK8. That of course would be coupled with sharing all the racers, karts, battle maps, and most importantly race tracks between the 2 titles. I would love to see something unique like that.
However, if all MK9 ends up as being nothing more than just more MK8, then I'd rather they just integrate all the new tracks, racers and karts into 8 as DLC and not make a separate new game.