Given his interest in foreign affairs, I could see JFK's career going the John Kerry route with a stint as Secretary of State under a Democratic President sometime in the 1970s after he returns to the Senate after the 1960 election. Another run for President in 1968 is certainly possible. 1964 seems rather unlikely, though that would depend greatly on how the first Nixon term went and on how willing JFK was to run for President in a year where he was also up for reelection to the Senate. JFK would have been 51 in 1968; assuming he had no Addison's issues, he would have still been a young candidate for President.