Something occurred to me in the shower this morning:
Toy Story 1 involves the well-structured crew of Andy's room be disrupted by the sudden arrival of a strange character from outer space. Toy Story 2 involves the characters travelling to the space invader's place of origin, where they discover hundreds more like him while gaining two new friends for their journey - a girl who is terrified of small, dark spaces and a non-human. Toy Story 3 eighty-sixes several beloved main characters right off the bat and dumps the rest in a gigantic prison for most of the film.
If Toy Story 4 happens, I predict it will be about a different, reconstructed Woody with several new features more reminiscent of an advanced, Buzz Lightyear-esque toy having to stop a shipment of defected Buzzes from arriving in America.