I would be absolutely happy with Brad Bird directing... "Ratatouille" being my favorite Pixar-movie next to "Toy Story 3", written by Michael Arndt. So I guess the pairing of the two could be "incredible" (pun intended).

Apart from writers/directors I wouldn't really mind seeing Johnston act in a more behind the scenes capacity, at ILM. Although Doug Chiang did some great concepts for Episode I, that DID in fact call back to Ralph McQuarrie, how about having Johnston as art director? I don't know if he is still interested in stuff like that, but it might just as well result in some OT-styled designs for the sequels.