heir presumptive 
noun (plural heirs presumptive)
An heir whose claim may be set aside by the birth of another heir. Compare with heir apparent.Still regarding himself as the heir presumptive to the French throne, he became the sworn enemy of the French regent, the Duke of Orleans....
- After considerable hesitation, not least by her father who feared that his daughter's marriage to the heir presumptive would make him unpopular, the couple were accepted at court.
- The impatience of the heir presumptive most notoriously surfaced at a breakfast hosted by the Guardian last November.