What would it take to ensure that the tradition of royals marrying royals from foreign royal houses continues to this day?
For example, instead of Prince Harry marrying Meghan Markle, he becomes engaged to a Princess of Spain or the Netherlands or Sweden, etc.
This type of marriage seemed to be common until just after WWII, Elizabeth and Philip (of Greece) are the last royal couple in Britain I can think of who married between the ruling royal houses of Europe (Windsor and Glücksburg). Many of the royal marriages in Britain in the second half of the 20th century were between those from the actual royal family with members of the British aristocracy or military, but how can we keep these marriages between the major royal houses of Europe, not just within the upper class of one country.
IMO, it would require saving the majority of the European monarchies from falling (possibly preventing the rise of communism), which would probably require a pre-1900 POD.