They've already used different actors for Buzz and Woody in other productions, so they probably already have someone in mind.
Patrick Warburton did Buzz in the Buzz Lightyear of Star Command cartoon series, and Jim Hanks (Tom's brother) has done Woody in the Buzz Lightyear movie and the Toy Story video games.
(And, really, where has Tim Allen been lately? ABC/Disney made him a wealthy man, he might feel the need to repay the favor.)
If Disney wants this done, they'll find the voices. The bigger concern is if they can find a story.