I still think Syrup is the only recurring character here I'd enjoy seeing anew. Every other villain served their purpose in their games, though I wouldn't be opposed to exploring the Shake King's dynamic of "Wario who fully indulged villainy" again. Perhaps a more fleshed-out reimagining of Shake It that better showcases his conquest would pique my interest.
Speaking of, is there really a possibility of the Shake King returning? First I'm hearing of it...