Relatively few individuals could fill that role. But here are three that spring to mind.
1) Jon Huntsman
A former governor of Utah, former ambassador to China, former ambassador to Singapore, and former U.S. trade representative, Huntsman currently serves as ambassador to Russia. Extremely well-versed in national security policy and fluent in Mandarin, Huntsman’s dual-experience in China and Russia would allow him to continue Mattis’ reforms towards better preparing America for great power struggles. Huntsman’s particular knowledge of and experience with the Chinese would be of instrumental value here. After all, China’s challenge to the U.S.-led international order will define questions of security for the foreseeable future. Yet, where Huntsman posseses Trump’s respect, he also retains the confidence of the Senate. That matters because senators will be looking for a Mattis replacement who they believe will offer competency and hard advice to the president. They will be rightly cautious against confirming someone who will simply tell Trump what he wants to hear.
2) Jack Keane
A retired four-star Army general and part-architect of President George W. Bush’s highly successful surge in Iraq, Keane is a Fox News contributor and a hawkish realist. Yet, like Mattis, Keane is a keen strategic thinker who shares the current secretary’s penchant for seizing the initiative. In a recent Fox interview, Keane honestly explained how China and Russia have now exceeded U.S. military capabilities in some areas. He knows the Defense Department inside out and would be able to go full throttle from day one. Keane also gets the importance of working with other levers of national power to pursue strategic effect. That would make him a good partner to Mike Pompeo’s State Department. I suspect the retired general’s tough-minded attitude and appreciation for discretion would earn the president’s favor.
3) Michele Flournoy
A centrist Democrat who served at the Pentagon under Bob Gates during the Obama administration, Flournoy is respected for her intellect, energy, and ability to work the national security bureaucracy out of its tendency towards lethargy. Would Flournoy serve under Trump? Perhaps. Would Trump appoint Flournoy? He would if he wants to consolidate the Senate towards a greater confidence that he is willing to seek honest advice. Flournoy’s arrival would also help balance out the more overt hawkishness of Pompeo and national security adviser John Bolton.