Most Roman emperors didn't have children, they adopted. Mary Beard writes that "The so-called Julio-Claudian ‘dynasty’ was cobbled together from adoptions, marriages, and last-minute fixes... it was a family only in the loosest sense." (SPQR, p.406). Augustus managed to convince the Roman elite that his claim to succession was more valid than Caes... See more
reddit.comWhy did Roman emperors mostly adopt, while European monarchs were obsessed with bloodlines?
Frank kings had rule over the other free men through detaining a magical power called Mund. That’s one point : Roman Emperors at least in the early times got power through law (though Augustus was divinised). Franks got theirs through intangible magic ; blood here is naturally important whereas adoption there was enough legally.