A well-known succession plan is male-preference primogeniture. There is a "reference ruler" who starts the dynasty. His oldest legitimate son inherits. If all his sons have no legitimate heirs, his oldest daughter inherits. Then do this recursively each generation down. And in OTL if your parliament have something inconvenient like James II in the way, just kick him out of power and rewrite the laws of succession to disqualify anyone of the wrong religion. Essentially primogeniture is searching the family tree delph first, with some modifications (valuing males put females lower in the order but they're still there, disqualifying Catholics is essentially introducing a new rule that prematurely prunes some branches the same way death would).
Roman law was different. The oldest son did not automatically inherit, the wealthy landowner decided who got the most land, and the other sons would sometimes get small leftover that still made them quite rich. The reason Nero was Claudius's heir was not because he was older than Britannicus but because Empress Agrippina the Younger convinced Claudius to favor Nero, which was very easy given that Brittanicus's mother fell out of favor of Claudius after having some extra martial fun.
So how to we get most of Western Europe and the Mediterranean to accept heir selection by kings as the primary method of heir selection and go to primogeniture as a fall back option if the king (or queen) should die without a heir? And if you're wondering how there can be a ruling Queen since "why would a King select a woman as his heir?" the answer is "A king wouldn't select a woman, even if liked his daughter he'd select his son in law. You'd have a ruling Queen when some monarch died with no sons or his oldest son was dead and had only daughters, or his oldest son had descendants but the senior branch of his oldest son's line ended in a girl"
Roman law was different. The oldest son did not automatically inherit, the wealthy landowner decided who got the most land, and the other sons would sometimes get small leftover that still made them quite rich. The reason Nero was Claudius's heir was not because he was older than Britannicus but because Empress Agrippina the Younger convinced Claudius to favor Nero, which was very easy given that Brittanicus's mother fell out of favor of Claudius after having some extra martial fun.
So how to we get most of Western Europe and the Mediterranean to accept heir selection by kings as the primary method of heir selection and go to primogeniture as a fall back option if the king (or queen) should die without a heir? And if you're wondering how there can be a ruling Queen since "why would a King select a woman as his heir?" the answer is "A king wouldn't select a woman, even if liked his daughter he'd select his son in law. You'd have a ruling Queen when some monarch died with no sons or his oldest son was dead and had only daughters, or his oldest son had descendants but the senior branch of his oldest son's line ended in a girl"