Can't wait for this.
I am very interested to see how they incorporate Slinky (as mentioned above), as well as other minor characters such as Wheezy and Lenny for that matter (voiced by Joe Ranft - r.i.p).
Some have suggested that they are planning to use archive footage - or sound-a-likes. I hope this is handled correctly.
The story is quite apt. As it references both the previous movies which are about the relationship between child and toy, and how the toys all have hang-ups about being loved and\or abandoned.
So there is plently of room for characterisation, rather than the (thankfully) abandoned Disney (Circle 7) version.
Plus it is part of the story-teams original vision of the continuation of the story (and end). And that is what I think everyone wants to see, as opposed to a tacked on 'modern' story as could have happened.