With Swedish patronymics, the most common thing I have found for an illegitimate child is to have a patronymic based on their father's first name (remember that the father was usually known by the mother and child even if the parents did not marry) or to use the same one that their mother used. I have very rarely seen a matronymic, perhaps because this would have made it very obvious to anyone that the person was illegitimate.
Since children were usually only recorded by first name until they left home, I track them forward until I find a last name before I create their Wikitree profiles. If I can't find a last name (for example, if they died in childhood) I will assume that it would be the same as what their siblings used. -Elisa