Lebron's two BEST destinations are Cleveland & Boston. Cleveland b/c he grew up there and is comfortable there. Boston b/c they have the best basketball fans in the world, and are the most successful basketball franchise in history. I'm not saying it's likely, or even possible (outside of a S&T, etc), but it's the best 
As interesting as it would be to see him in Green, I think Lebron needs to find his Own Destiny.
He would be just another player if he came to Boston. He would certainly be a Great Player in Green, I think, but just the thought of him trying to forge his own destiny in Boston behind the works of Bird, Cousy, Heinsohn, Parish, Russell, Mchale, Pierce, KG, Ray, etc.....
He just needs to be somewhere that he can make a name for himself. It would be awfully hard to do that in Boston. Too much for him to live up to, even though he'd have a Great Start in Perk and Rondo (after KG, Ray and Pierce move on).