This should be the plot for Frozen 3:
Arendelle people want Queen Anna to wed Prince from a nearby kingdom to strengthen the relationship and for resources/trades etc..
The Prince from that Kingdom is not evil at all, he is actually very nice and Anna starts questioning her love for Kristoff.
Elsa dives into the unknown and questions her existence. She is adopted and that shatters her illusion which makes her move away from Anna.
They can also play on who Kristoff is actually.. He wad adopted by trolls but how come he ended up there.