UPDATE: After publishing it was later confirmed that the Toy Story sections of KH3 would take place after the events of Toy Story 2. This doesn’t completely rule out some of these villains, but certainly lessens their chances.
Emperor Zurg is the very definition of a villain, at least at first. This arch nemesis of Buzz Lightyear certainly looks imposing, as his mission is to thwart Star Command at every instance. Zurg’s villainy would make him easily swayed by the likes of Xehanort and the Heartless. You can just imagine the perfect boss battle with Zurg, atop the elevator like in Toy Story 2 as different obstacles whiz past you going up and down the shaft.
Of course, Zurg would have his own personal army of toy-based Heartless to contend with while he blasts away at you with his gun as well. He seems like the most likely choice for a villain from the Toy Story world, and would serve as an interesting force to contend with throughout your time in Kingdom Hearts III.