I'm wondering if we're seeing the beginning of an 'element quest'. Somehow, Elsa learns that she is not the only 'element witch' in the world. There are many people with elemental powers and she goes to seek them out. Possibly there is information brought to her attention that they are either a threat or needed to avert a threat.
I wonder if the crystal runes that we see hovering in front of Anna are in the Trolls' language and it turns out that Elsa is creating them whilst asleep in response to a prophetic dream. The message is, as is always the case in these matters: "Doom is coming! The Ice Queen must..."
So, Elsa has to go off and find all those who can manipulate the elements. We have seen what appears to be an air wielder and a fire wielder.
The fire wielder is definitely being pushed as the antagonist but, based on the first scene in the trailer and the semi-canonical Frozen section of Kingdom Hearts 3, whether Elsa is also going to learn her parents' fate and just who was responsible for it. I'm thinking that she's trying to reach the wreck of her parents ship.
Maybe there is either going to be further films or a spin-off show on Disney+ following Elsa's quest further and also her adventures in statecraft back home in Arendelle. Similar to how Riders of Berk and Race to the Edge filled in the gaps between the first two H2TYD films.