If she wins the presidency, she'll likely run again.
Hell, she might run again if she loses. I find it weird that someone who has the advantage Hillary currently has isn't seen as a credible contender for 2020 against a Republican incumbent.
I don't recall Tom Dewey running again in 1952 despite Truman's unpopularity. Did he not have an advantage?
Sorry but no, between how old she is now, the media firestorm, and the fact that only Nixon and Cleveland ever came back from exile....it's just not happening.