The other easter egg is the music. The rendition is a remix of sorts of the original Wario Land theme from the first two titles (the part with the brass instruments near the beginning is straight from the theme itself).
Wario in his Gameboy colors, from Mario Golf 64:
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-Greenhorn forest from Wario World got a remix in DDR Mario mix.
-The Wario land 3 theme plays during Warios trophy celepration in power tennis.
The Wario courths, shoulder charge in world tour,sleeping animation/wild swing ding in brawl ans the arcade Diamond city racetracks, have their own threafs already, so if youre courius check them out!