Check out the full, extra-wide version of the key art Nintendo distributed:
https://mario.wiki.gallery/images/8/8b/MKWorld_key_artwork_no_logo.png
The P-Switch is on the left side, on a ledge above the food truck. I have seen people talk about the P-Switch showing up briefly in Treehouse footage, but not the fact that it's shown off in official artwork.
As for a secret character … If you count the racers, there are exactly 24 … But 2 of them are Goombas! I'm guessing one of them will be swapped for a different character in the final artwork.
Most of you probably saw the young Pauline character (Pauline Jr.?) that was leaked by Nintendo Korea in alternate DK Bananza key art. So we have a very recent example of Nintendo hiding a character in this manner.
The easiest guess is that they're also hiding young Pauline in Mario Kart World. Or perhaps it's Diddy Kong with a Bananza redesign. Or perhaps it's someone we could never guess…
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