I've just realised some things about Toy Story. (SPOLIERS)
TS2 foreshadows TS3:
In the scene of Woody's nightmare, Woody is dropped by his owner Andy and he falls into the trash can, where the trash anthropomorphically grabs him and stops him from escaping. (Note that all the toys are essentially anthropomorphic too) In TS3, all the toys end up at the recycling centre, and nearly get incinerated because they get pushed into the trash bins.
Also, each movie has a theme, which all centre around fear - a fear that is relevant to the life of a toy. Ultimately, children and adults have these fears too.
TS1: Replacement. Woody fears being replaced as the alpha toy and becomes envious of the new and more popular Buzz Lightyear. (Queen Bee in Mean Girls, anyone? Also, the fact they are called 'plastics'?)
Also, fear of being cannibalised and destroyed by Sid (another foreshadowing to TS3 with Sid as the bin man)
Also, fear of being lost & not being able to go home.
TS2: Purposelessness/Being forgotten. Woody accidentally ends up being put up for sale amongst forgotten toys, and is stolen by the greedy collector Al, where he meets Jesse & others, who were also forgotten about and abandoned by Emily. Al was counting on the toys being remembered about forever and so regaining purpose.
TS3: Suffering (through separation, imprisonment and destruction). As Andy is now going to college, the toys are split into staying with Andy, loft storage or donations. The toys that go to the day care centre aren't treated with respect, imprisoned by Lotso, and are abused by the children. They are then nearly permanently destroyed by incineration, before being donated by Andy to the next generation of children.
Also, Baby, the clown and Lotso are originally separated from their owner, only to be replaced by the same toys. This shows the evil path of transforming personal suffering into the suffering of others.
So which fear will they have in TS4?