So, what I expect or hope is, in regards to Elsa,as she learned to love herself with her powers and be proud of them, the most emotional step or next biggest burden would be if
someone were to steal the magic powers of all the elements (including Elsa's powers) to take over or something (could work, I mean if you can give Elsa powers, you can also take them away

). This would lead to a new side of Elsa being revealed, where she has to come along without powers for the first time, and thus the important desperate situation of whether Elsa, now that she has been giving the opportunity to be like a normal person, and this then-impossible desire to fully belong in society and not have to feel different, while having to wonder what makes her special as a person without her powers.
Therefore, I would like
Anna to be in a hardcore 100% Queen boss mode and this time it is her turn to protect and fight for her sister (instead of Anna having to chase after her for 2 movies) and make it her goal as Queen to challenge the evil king or boss who stole the powers and make it her goal with her Arendellian and Northuldra army to return the powers to their rightful owners (including Elsa), while not allowing Elsa to join the war/fight for her saftey.
It would also be good because
Anna and Elsa's roles would be reversed, since Anna is aware that Elsa is now the helpless one who needs to be protected and the one who stays behind and gets shut out of Anna (to protect her), who is in action mode.
Maybe you could turn
Hans or one of his brothers as the big villain who gathers the forces (which I would VERY much like, since the worldbuilding would thus be expanded and
we could FINALLY see the Southern Isles for once). Hans would realize how wrong this is, apologize to Anna and Elsa, try to regain their trust, team up with Anna and face his brothers in the climax and somehow have a character arc.
Regarding character arcs, you could
reuse Kristoff's ACTUAL story arc from frozen 2 since Kristoff is now engaged to the QUEEN of Arendelle and therefore all eyes are on him as well and he feels uncomfortable in his position, hiding his feelings from Anna.
Since Anna leads an army in my head canon, she'll wear a cool knight-like outfit (like in Mirrorverse, only something that screams more queen or leader.) And Olaf in a mini knight armor would be fun, though that might not make sense since he theoretically wouldn't exist when Elsa loses her powers, but that's not my problem but Disney's.
(I really hope that Kristoff will have a reason to exist this time (not like in the 2nd part)
And overall, I hope it will be WAY more darker and serious in tone, like I expected by the first teaser of 2
And
another sequel Idea I thought, was,that the focus would be on Kristoff this time, as Elsa might see a vision of Kristoffs past in Atholholan or whatever its called, where then he sets on a journey with Anna, Elsa and the trolls to find his biological family, where he has to decide wheter he stays with Anna and his troll family in Arendelle or his biological one (of course he stays in Arendelle)
The
last idea I had was based on this well-known theory that the trolls are evil and have manipulated Hans' memories. Elsa could discover this sequence in the ice cave and confront them. The trolls planned this from the beginning so that Kristoff could hook Anna and he would then become future king of Arendelle at Anna's side (and the trolls, as his family, would automatically gain power as well), which is why they advised Elsa to follow the voice in part 2. and there would then be an epic battle between the forest spirits and Elsa vs the trolls.
But of all I like the first idea the most.