First of all he is scheduled to make like $7 mil next year from the Broncos. If he takes another coaching job somewhere, the league will not make the Broncos give him that money. For that reason, its entirely possible he dosent coach again until his contract is up with the Broncos. He would basically be working for less money than he could make by not working. But, its possible the guy would want to work and decide to do it anyway.
On top of that Shanahan will be no one's OC. He is a head coach. Teams will be lining up for his services when he decides he wants to coach again. Personally, I would love to have him as our head coach.