Since MK8 on the switch will be a juiced up version of the Wii U game (if the cover art gets it right), The new tracks will most likely include the 16 extra tracks we've seen in the Wii U version, this will probably mean that they will add two more cups, and maybe even a possibility to combine the online gaming of the Wii U users (excluding the new tracks). Nintendo could even decide to release an update for the Wii U version with two more cups.

This will also leave a door open to see a new MK for the switch as well, lately it has been a tradition that Nintendo releases a new MK game every 3 years. The year 2017 will be the 25th anniversary, so I'm really hoping for some sort of Greatest Hits, Grand Prix, All Stars for the official switch version.

But does a rerelease of a Wii U game also mean that the used engines on the Wii U are "easily" ported to the Switch?

I'll guess we have to be patience to know what tricks Nintendo will have up it sleeve.