I realize how broad this hypothetical is. That's one reason for the set limit to only three choices. I can't help but think that President Spock would have all sorts of absolutely brilliant mandates that he would be starchily adamant about, implementing them immediately,and, with very little forethought ! It would be only logical ! For me,..first he would buy a nice looking suit and dawn a blueish green tie. Second,... loss the funky out of date hair do ! And third,...well third,...that's the clincher,...ain't it ? How 'bout increase drone activity over the Mid-East by 1,000 fold !!!!! I betcha that would make a pile of assholes shut the fuck up !! Anyway,.. that's my take.
The republicans would NEVER work with him. He would be way too logical for ANY of them!
Democrats would not work with him either...for the same reason.
The Constitution dictates that only American born shall be elected President. That takes care of Mr. Spock! HA!
A candidate for president chooses Spock as a running mate, gets elected, and dies in office.
A US president inspired by Spock-like logical thinking would, at least, stop fossil fuel subsidies, stop drone attacks, and explain to the people, in an address, that they are capable of the greatness necessary to sustainably manage their home planet. Planet-scale thinking might be unfamiliar and new, but human beings are flexible and adaptable, able to learn in order to survive.
Such a president could also logically analyze the psychology of the American people, to figure out how to inspire us to rise to planetary-scale thinking.
(As for Mr. Spock himself, we could still amend the Constitution to allow naturalized citizens to become president.)