Former Marine Gen. James Mattis keeps being mentioned  as a possible nominee for Secretary of Defense:
The clearest consensus inside the transition team is for Gen. Mattis, a former war commander who has long voiced concerns about the security threat posed by Iran.
Choosing Mattis would be fairly unusual and I believe it would be the first time in decades that a recently retired general was named to the position. Doing that would require  a special bill to permit a recently retired military officer to serve as Defense Secretary. This was the same issue that cropped up when Flynn’s name was being mentioned in connection with the job. The law prohibits a newly retired officer from taking the job for seven years, and Mattis retired in 2013, so there would need to be an exemption passed by Congress:
If precedent holds, Congress would have to pass a law to exempt him from the requirement.
That’s what Congress did in 1950, when President Harry Truman nominated Gen. George C. Marshall for defense secretary. At the time, officers had to be out of active duty for at least 10 years before heading the Department of Defense.
Assuming this hurdle could be overcome, picking Mattis would continue the pattern of filling top national security posts with people fixated on and hostile to Iran. Mattis identified  Iran as “the single most enduring threat to stability and peace in the Middle East” in a speech in April. He also asserted that “Iran is not an enemy of ISIS,” which would come as news to both. He went on say this:
I would just point out one question for you to look into. What is the one country in the Middle East that has not been attacked by ISIS? One. And it’s Iran. That is just more than happenstance, I’m sure.
That happens to be false , but even if the claim were true it is a bizarre, conspiratorial sort of reasoning to use. If a country isn’t being attacked by a terrorist group, that doesn’t mean that its government is somehow in league with them or on the same side, and it doesn’t say much for Mattis that he thinks there is something significant about this “fact” that happens to be wrong. Iran hawks often seem determined not to accept that Iran is opposed to and actively fighting against  ISIS and other jihadist groups like them, and so they will try to explain away the obvious antagonism between them. It doesn’t bode well for U.S. policy towards Iran and the region as a whole if Trump picks Mattis.