[Shiro has a feeling Mikoto lacks his powers in Vatheon, but he's not going to speculate on that out loud.]

No, no, of course not. There have been many Kings since the discovery of the Slate. Eventually some one else is gifted with the power. Whether or not they return to the previous clan or not is their decision though. Some Kings have even had no clansmen, or waited a very long time to accept people in to their clan.