Pretty obvious for me to start talking about Toy Story. Well what would come to a third installment of this series, I'd pick up a plot where Andy has either grown up or gotten interest for something else than toys (video games, ect.) and the his toys would need to handle with that.
Character developement: we saw at the end of TS2 Buzz and Jessie getting know each other. Well there's a ready sub-plot about their relationship. ::grin::