heir presumptive has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is heir apparent.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a person who expects to inherit but whose right can be defeated by the birth of a nearer relative
Synonyms, antonyms and the broader and narrower relations for heir presumptive come from Princeton University’s WordNet. Full sources and licences.