Cannot say much about royal inheritance in a European styled setting, but in Japan, a given lord, whether a daimyo, samurai lord or shogun merely needs to adopt his chosen replacement, to take office upon his death. The title of shogun was conferred unto Minamoto Yoritomo by the emperor. All subsequent shogun houses had to be a Minamoto in order to take title, up to and including Tokugawa Ieyasu, except Ieyasu had to be adopted into the Minamoto line, who had long not been the active shogun, though still carrying the bloodline exclusive to shogun status, so he had to find a living heir and be adopted by that person to gain the title. But this same process occurred across the bureacracy of Japan. You needn't marry into a family, nor be an actual descendant, you just need to be legally adopted and you qualify.
Now imperial inheritance is more like Europe being father to eldest son, normally. However, one emperor only had daughters, and his eldest daughter was married to her chosen husband, who had to be adopted by the emperor to gain title, but still required marrying the emperor's daughter to hold title. Sometime in 6th century of Japan, Suiko was the only daughter of the emperor, and was married to the new emperor, however, he died only a few short years later, and Suiko opted not to remarry and gained the title Mikado or emperor, and was the first ruling female emperor of Japan - one of three, though the title "empress" doesn't exist.
In Celtic society, you have tannistry, which meant that the ablest member of the clan is it's clan leader and voted by the rest of the clan into the position, but it isn't a lifetime position, at any point if the majority of the clan feels the clan leader (and could be a king), should be replaced by vote. they are. However, if the king/clan leader dies, then the next in line is the next ablest member of the clan. Which could be the king's son, but just as likely his brother or sister, or cousin, or nephew or niece. So whoever is the best candidate is replaced, though the king's family name still holds title, it just isn't automatically your eldest son, like it is in non Celtic Europe.