I would guess an imperial match.
According to Vivian Etting's biography of Margrete I, the Kalmar kingdoms had been largely isolated from European affairs for several decades and had come to be an area of fascination for outsiders. That is to say, with England and France kicking the shit out of each other for the better part of a century, Castile and Portugal in a near-continuous state of war, the emperor dealing with constant revolts and challenges to his authority, the Visconti slowly conquering and subjugating their neighbors in northern Italy, and the two Sicilian kingdoms a total mess, the various crowned heads of Europe had come to look at Denmark, Norway and Sweden as peaceful and prosperous and wondered how Margrete was able to hold it all together. If Etting is right in this description of Margrete's reputation, then Erik probably could have secured any bride he wanted,
if not for the fact that Margrete wouldn't agree to any marriage that brought her kingdoms into foreign wars. (Erik and Philippa's OTL marriage negotiations dragged on for four years -- and the early concurrent negotiations for Henry and Catherine broke down entirely -- because Margrete would not concede to English demands that her kingdoms provide aid in a war against France.)
So, when trying to find marriages with people who are powerful enough to merit their daughter being made a queen, but who are willing to concede that you will not aid them militarily, you end up with a very short list. But I think there are some options:
- Rupert III of the Palatinate had Agnes
- Rudfolf III of Saxony had Scholastica
Wedding their daughters to a crowned head of Europe would be a huge boon for their families, and as prince-electors, it would give Margrete a network of connections across the empire.
Seeking a "conventional great power" would make Portugal and, especially, Scotland very odd choices. Portugal was a small and poor kingdom on the edge of the Atlantic at this time and Scotland a backwater with few continental connections outside of France. It wouldn't be until the reign of James III that the Scottish parliament would even concern itself with raising the dignity of the king, passing legislation to give him the power over church appointments and notaries -- areas of basic governance that kingdoms like England and France had concerned themselves with centuries earlier.