There is no way players like Shaq or Dampier accepts to sign as the 6th big. If Spurs sign a vet 6th big, it will be a marginal player. I'm not even sure a player like Rasho would agreed to be a 6th man.
Personally, I rather see Spurs signing a young player as 6th big and letting him work in Austin. Richards could have been that guy but his shoulder surgery rules him out.

