Personally Elisif and Ulfric should be IMO. Elisif for a male player and Ulfric for a female(Nord only cause of his prejudicial tendencies).
Reasoning: They are two of the most powerful people in Skyrim, and they are both single. Sure Elisif is a widow but she can remarry.
Also characters who are married, but become single through.... certain circumstances, should be allowed to remarry. Like Adrianne.