Donner as Rudolph's Mother

So we all know that in the 1964 Rudolph cartoon, Donner is stated to be the parent of Rudolph, but in the 1939 book, Rudolph's parents are never actually mentioned (it even never implies Rudolph came from the North Pole in the first place). But the idea of Donner being Rudolph's parent is kind of feels staple in Rudolph lore (Donner is actually female in my interpretation, despite the name), so my question is, when the Rudolph book becomes public domain, would you be able to have Rudolph as Donner's son? Would that be running too close to copyright, or are potential familial relationships between two public domain characters something too trivial to be copyrightable?