The title would be Mister or Madame President, in the US we've already established Madame as the proper prefix to Speaker of the House and Cabinet Secretary and refer to foreign female presidents as such. "Madame Speaker" for Nancy Pelosi in her tenure, Madame Secretary for the various female cabinet members. Also we generally don't mix and match elected, military, religious, and academic titles. Dr. Chu, Sec. energy, is addressed either as Doctor, Secretary or Mr. Secretary, not Dr. Secretary.



He would be Mr. President, as Wilson was addressed, Dr. is the appropriate title for a Ph.D. or M.D. Or I suppose J.D., which President Obama, Clinton, and Ford had and probably some others, no one uses it though, law not being an abnormal field to spawn politicians I'm sure many have. We've also had several generals serving as president and as cabinet members or cabinet-level, and they are often refered to as General (or Admiral) but never when POTUS because we have this thing about military uniforms on the president.

More importantly, the President of the United States is not the only president, not even in the US gov't. The proper mode of address to the person presiding over any number of groups is "Mr/Mrs/Madam" President, we're just lazy people, same reason we rarely refer to our ambassadors as "Your Excellency". Big thing though is we don't mix titles often, people are addressed by the highest of a given type they got (general rather than colonel or major) and/or the active thing they are doing that might trump it. No Dr. General for highest ranking military medical personnel, nor Dr. Governor or Rev. Gov. of which there have been some, though governor-general has been used for military governors.



If it were old Bill, I'm sure Mr. President would continue to be used, though he joked about being the "First Laddie" if I recall. For anyone else likely their personal title if they had one, and most women near the top of things these days tend to have professional husbands. First Lady was adapted with time, early on some of the woman had themselves addressed as Mrs. President, Madame President, and Mrs. Presidentress, and I gather Your Highness. There is no established precedent yet, just as there wasn't for first lady, it will depend on the guy, and if he already holds a title of his own he's likely to prefer that or view it as a humorous irrelevance, the top runner is "First Gentlemen". This is something of a holding action though, because it ignores unmarried partners, not mention same-sex, though the former is still the more probable. I'm sure if an unmarried president were dating/engaged to someone the media would find out and figure out an address. Obviously the mistress of a married president is never titled except derogatorily but I'd imagine people would want to slap on some respectful title the the fiancee of a president.