Maybe a list of capable assistants doesn't count as one candidate. But a number of really good head coaches have come from the inexperienced ranks over the last couple of years.
Here's a good start:
Mike Zimmer, Bengals defensive coordinator
A player's coach, Zimmer has gotten the most out of every defense he's coached. Eventually, an owner is going to hand the keys over to Zimmer. Do the Chargers need a change in personality, however?
Gus Bradley, Seahawks defensive coordinator
The work Bradley has done with a no-name defense is astounding. Given a rag-tag mix of players, Bradley has turned Seattle into one of the NFL's most feared defenses. He's also developed young players, such as Richard Sherman, Brandon Browner, Kam Chancellor and K.J. Wright, into key contributors early in their NFL careers.
Todd Bowles, Eagles defensive coordinator
You just watch what Bowles, the Dolphins former assistant head coach, does with the Eagles defense. That unit will be much better, and it make Bowles one of the hot assistants for 2013 and beyond. He's a rising star that needed this chance.
Kyle Shanahan, Redskins offensive coordinator
Shanahan has coaching pedigree and a building head coaching interest. His work with Robert Griffin III and the Redskins offense in 2012 has been a revelation, but it all goes to show how prepared and smart Shanahan is as a play-caller. However, is he the right fit with Rivers?
Jay Gruden, Bengals offensive coordinator
Jon's younger brother, Jay Gruden has done exceptional work with young quarterback Andy Dalton in Cincinnati. He's not as over-the-top as his older brother, either. You'd have to think that it's just a matter of time before there's a team asking Gruden to be its head coach.