I agree with your assessment that Sheilds is a very attractive offer to many teams, especially considering the cost right now for pitchers. Which is why I think the Padres will be smart about the situation and wait. They can just start a bidding war form the teams that missed out on the big name pitchers (Giants/Dodgers, Cardinals, Jays and others). I can see them getting more than a 40-year old knuckleballer who may retire at season's end and a pitcher who has taken a step back to the point of not even being considered a part of a big league rotation.
If the Padres even agree to eat some $$ in his contract that makes it even more attractive and could interest other teams to join the bidding as well (*ahem* Boston, cause they still have prospects left and DD is somewhat dumn). If the Jays hope to make a competitive offer for Sheilds I imagine it will start with one of Pompey or Osuna and go up from there.