There are many variants of succession. There is elective, absolute primogeniture, male-preference primogeniture, salic-law, seniority...
And then there is "Ummm, not sure"
So the challenge is, with a POD 900 or later that doesn't butterfly Henry II of England's ascension (so no Cunte starts a super dynasty), make Denmark a whole kingdom (not having its territoriy chipped by neighbors) but it's succession law is "Ummm... someone related to the last monarch" with not even an elective system to sort it out.
Instead, by 1500 there is still no codified succession and there had been inheritances by oldest sons, middle sons, oldest daughters, brothers, nephews, son-in-laws, grandsons, and great-grandsons. Therese have been decided by willing the kingdom to a heir, ad-hoc meetings by royal family members followed by agreements, and sometimes a fight for Copenhagen as a... tie breaker.
Bonus points if the Sweden has male-preference primogeniture and there were three personal unions, with 2 of them dissolving not by Swedish complaints, but by the royal family picking a different heir to Denmark-Norway than the Swedish one. "Yeah, the Swedish crown goes to the princess, but we think her uncle will make more headway in Denmark."