Filling out this Seneca Bridgerton survey gave me ideas... Basically, if Seneca had a squib child, she'd actually be 'merciful' and have them obliviated and then arrange for the child to be adopted by muggles. Which gave birth to this idea:

A squib character who is actually the "deceased" child of a pureblooded family. They were obliviated so they wouldn't remember who they actually are and grew up as a muggle, but recently they started getting their memories back...

In a similar vein, a character whose secret profession is helping prejudiced magical families "get rid" of their squib children. They find suitable muggle homes for the child and take care of the obliviating process.