uaigrynolraitru lujvo

x1 is the future (but not current) regent of x2 by standard x3.

Can apply to an heir-apparent, or their heir-apparent, or in a retrospective to a person who becomes regent in a previously unforeseen manner. The "future" part of the description "once and future king" in reference to King Arthur would be sufficient to make Arthur a supposed uaigrynolraitru.