For an actual theatrical movie, it just wouldn't seem right. I mean, the Toy Story Trilogy wasn't just about good toys vs. evil toys. It was much more than that. And making a sequel just for that wouldn't seem like a good idea. Now, I know it doesn't have to be just about that, but what else can it be about? After SPOILER ALERT! they all get passed on to Bonnie there pretty much isn't any other story to tell about them. Once Lotso or Stinky Pete comes back for revenge, what could they possibly do? Defeat them again?
It's not just about good vs. evil. The new movie would never about just that. And when Lotso and Stinky Pete come back, yes, they do have to defeat them again.