“I am aware of the fan requests for Pixar characters,” said Tetsuya Nomura, Director of the Kingdom Hearts series, in an E3 interview. “I will do my best. Maybe one day Pixar characters will be featured in the Kingdom Hearts series.”
A Toy Story world could mesh with the series and so could Finding Nemo. It’s Nemo or Atlantis, if Square Enix wants to use Mer-Sora without resorting to The Little Mermaid again.