Someone like Sherrod Brown or Tammy Baldwin. Midwestern progressives with establishment connections. Cory Booker has no chance with the activist base (Despite the borderline Republican beliefs of Blue Dog Moderate, who seems fine with Democrats being right-wingers). Same goes for people like Manchin (who's nomination WILL trigger a left-wing third party challenge).
But even then, it'd be an uphill battle for them. There are always going to be unsatisfied people.
Sanders also brought an interesting coalition of the Democratic doves, activists, progressives, democratic socialists, blue dogs, libertarians(particularly on the Federal Reserve, guns, and privacy/spying), and nationalists, most of which are usually at opposite ends from each other.
