It was a clever ending on their part. It left it open for a sequel, sure, but it didn't do it in such an obvious fashion that other films do that leave you feeling a bit disapointed how obvious it is they want to stretch or make a franchise.
They actually gave it some real closure by passing over the toys to Bonnie(?). If they make no more films then its a nice "ahh they got passed on and we're loved once more" while like you say they could indeed make a fourth with Bonnie(?) as the new Andy.
As a film I'm surprised it left me feeling as i did. In reality the story line was not a whole lot different to the second. "Toys get taken away from home, toys split up, toys decide they need to pull together and help one another get back to Andy." But it was just so well excucted and handled so well that it filled exactly the cushy hole you hoped it would when you entered the cinema.
Me and my girlfriend were in creases when the baby doll was first introduced!
