The Tribune Company actually posted the position on their website. It says: "Preferred candidate will have played with the Chicago Cubs — or played major league baseball with previous broadcast experience as a game analyst."
In his column today, Paul Sullivan lists the candidates that would fit that description: Mark Grace, Rick Sutcliffe, Dan Plesac, Eric Karros, Gary Matthews and Mitch Williams.
I'd add Keith Moreland and Dave Otto to the list too.
My top choice would be Suttcliffe, though I think he won't take the gig because he has a pretty sweet gig with ESPN. My second choice is Plesac. He's actually quite good.