I wouldn't mind him as Secretary of State....
Unless Trump and Romney share the same view of the world, risks and opportunities, I wouldn't turn him loose as a free agent.
For example, I believe Trump is pragmatically concerned about radical islam. If Romney is a kumbaya naïve do-gooder, I would not expect them to agree on this aspect of foreign policy.
Trump is probably a supporter of Brexit. Romney perhaps not so.
But if they agree on foreign policy, Romney is a man of stature and commands respect. After all, he was Republican Governator of Massachusettsistan, and ran twice for President.