Andy's Honeymoon
Prospector says to Woody in Toy Story 2, "Do you really think Andy is going to take you with him to college or on his honeymoon?" Well, the "college" part of that line actually played out in Toy Story 3, so would the "honeymoon" be the plot of Toy Story 4? Maybe Bonnie takes the toys with her on a cruise ship, and the toys find Andy and his new wife and try to reunite themselves with him. Toy Story 5 could then be about Andy and his kids playing with Woody and Buzz and the other toys.