Frozen 2: Boreal Destinies

Set after the events of Frozen and Frozen Fever, what if after Duke Weselton was banished from Arendelle, he ran back to Weselton with his tail between his legs, and reported his failure to his ominous master, who then decides to use his son to take over the throne? And what if said son was an old friend of Elsa's who she'd forgotten about? Better summary inside! Elsa/OC R/R!