Rapunzel, Eugene, and Cassandra were obviously meant to be in a triad polyamorous relationship. Here's their interactions off screen and their thoughts during key moments of the show. Rapunzel's individual relationships with Cass and Eugene develop first, then later Cass and Eugene warm to each other. Eventually, they synthesize their relationships into a triad.
How much more can Elsa's power grow once she steps into her role as the fifth spirit? How does Arendelle respond to Elsa's abdication and Anna's coronation? Set during the last few minutes of Frozen 2 and the months afterwards.