Attempt to reinstate the Gaelic heritage of the nation. Arrange a marital alliance to the daughter of the current Lord of the Isles. This might buy you favour with other Gaelic lords and may reduce the threat from these western frontiers.
Find an attractive, young European bride with good bloodlines. As King of Scotland you have responsibility to secure the Stewart throne, weakened since your father's assassination.
Which card will you play?
Not a good idea. The Douglas family is powerful enough as it is, without your bloodlines crossing. You want to strengthen the Stewart line, not the Douglas'. You also don't want to upset the rival families. Try and avoid this option at all costs!