kronprinco

Two words you already have. Krono at 1,413 is a crown, princo at 268 is a prince, and glued together they name the prince who is going to get the crown. Esperanto builds compounds by putting the modifier first and the head last, so the second half is what the thing IS and the first half tells you which kind: a kronprinco is a prince, of the crown-inheriting sort. Kronprincino is the female equivalent and needs no new root either.

It is not the same word as reĝido. That page makes the point from its own side and it is worth having both halves. Reĝido is built from reĝo plus the offspring suffix -id-, so it says BORN: this person is the king's child, and every one of a king's six sons is a reĝido. Kronprinco says NEXT: exactly one person at a time is the heir, and which person it is depends on succession law rather than on birth. A kronprinco need not even be a reĝido - a brother, a nephew or a cousin can hold the position when there is no son. And princo on its own says neither; it is the rank, and you can hold it by title, by marriage or by grant.

The word arrived with a specific era attached. It is a German-shaped compound, and in Esperanto's source period the Kronprinz was a headline figure across the imperial courts of Europe. That is why the word sits at rank 2,888 in a corpus from 1887 to 1930 and would sit far lower in one drawn from today. The reĝlando it points at is the thing he is waiting for, and the trono at 2,175 is where he sits when the waiting is over.