Shikamaru Ninja
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As LordKano pointed out, it's in the Switch trailer. Yoshi Circuit (GC) is a DLC track in Mario Kart 8. You can see King Boo in the same trailer, but I would suspect that some DLC was held back because of the Wii U's apparent hardships. Emily Rogers also posted this on her blog page, and she's been mostly on point about the Switch information she's shared. After that, Hideki Konno (Current Producer for the Mario Kart Series) has had a directorial role in Super Mario Run recently, and it's not clear who would have filled that position. Everything points to a Mario Kart 8: Deluxe Edition. It'll be a while before we see Mario Kart 9.
Just want to point out that Hideki Konno is the manager of "Smart Device Production Group" which works on all the mobile games alongside the team the IP corresponds too (Sakamoto for Miitomo / Tezuka and Kimura for Super Mario Run).
Mario Kart 8 has another producer (Yasuyuki Oyagi) and director (Kosuke Yabuki) who worked on 7 and 8, and will continue the franchise beyond just fine.