Many people have tried to dissect the movie's spectacular success and to explain what makes it so different from other Disney movies. The idea that it is a game changer in terms of story telling, feminism, and breaking with the princess stereotype is probably bollocks. As others have pointed out, Mulan already saved the day without any real help from her love interest, and Merida did not even want a love interest. One might also mention that the two Frozen princesses are actually distressingly stereotypical in their design, unrealistically small compared to the male characters and unrealistically thin.
Apart from the stunning snow and ice visuals, what I think is really somewhat special about this movie is that it deals with a different kind of darkness or danger than your average Disney princess movie. Not in every case but usually there is an obvious goody-two-shoes heroine with at best very minor flaws on the one side, and a cartoonishly exaggerated villain on the other side.
What, really, are the flaws and personal issues of any of the other famous Disney princesses? Snow White, Aurora, Cinderella, Belle, Mulan, Rapunzel: nothing I'd be aware of. I mean, naivete doesn't really count because it would be harmless unless exploited by the bad guys, so all the bad things happening are squarely the latters' fault. Ariel does something quite foolish although she should know better but well, her father had left her no other options. Ludicrously, Tiana's 'character flaw' is that she believes that the way to achieve your dreams is hard work instead of kissing a prince (boooh, I guess?). I may be overlooking some counter-examples because I have obviously not seen every Disney movie, but really we seem to be looking at a bunch of Mary Sues here.
Conversely, the villains are generally unrealistic, mustache-twirling evildoers without any redeeming qualities whatsoever, and often enough without any clear motivation for their behaviour beyond all-purpose evilness. Seriously, look at that voodoo guy in The Princess and the Frog, or at Ursula the Sea-Witch, or at the Hun leader in Mulan; they are so over the top that even a four year old will know that such people and the problems caused by them are rarely a concern in real life. Alternatively, there might be a curse (as in Beauty and the Beast or in Brave) which means that the viewer does not have a villain to hate, but it also means that there is nobody to feel sorry for if the curse is broken. There is never any doubt who the good guys are, what is wrong, how to put things right (defeat the villain or break the curse, duh), and that the happy ending will ultimately be achieved.
Only recently do they seem to go for more complicated, more conflicted heroes. Merida was brash and stubborn and nearly brought ruin upon her family, and the point of the story was that she had to acknowledge her mistakes to break the resulting curse. (Still, again one could argue that without the idiot witch nothing really serious would have happened in the first place.)
(Spoilers ahead, in the unlikely case that anybody doesn't know the story yet.)
Frozen, if you think about it, has an even darker outlook. There is no powerful, obviously evil sorceress or war leader coming in to trash everything for the giggles; to the degree that there are villains, they are actually considerably less powerful than the protagonists, and all the bad stuff would most probably have happened without them. There is not even a curse that has to be broken. Instead, the entire problem reduces to the fact that Elsa, by far the most powerful of all characters both politically and magically, is traumatized and suffering from massive anxiety issues, and that she causes suffering through an accident.
That makes everything much more complicated. Not only is that much harder a problem to overcome - you can't just kill her off like a cartoon villain, nor would it be convincing to have her issues resolved by simply regretting what happened; it also means that what damage is done during the main action is done essentially by a person the viewer is rooting for. What is more, it is unlikely that she is entirely happy and free of regrets even once the movie is over, considering all that she personally lost and foregone as a result of not mastering her powers a few years earlier.
That is what makes this movie so different and thus much less boring than the standard fare. Now how would a sequel deal with that situation?
Elsa is clearly the character that viewers find most interesting, and the happy end of the Frozen movie was that she had overcome her anxiety issues and gained full control of her powers. The thing is, full control of her powers makes her ridiculously powerful. It would not be an exaggeration to say that she is - presumably with the exception of immortality - equivalent to a god from a polytheistic religion. And that means that it is very hard to conceive of a plot for a sequel that could not immediately be resolved by her, that would not have even a kindergarten child asking "why didn't Elsa just do X in the second scene of the movie?"
She can freeze water by touching it, and presumably now also unfreeze it. She can create small and large ice structures and even animated snowmen by waving her arms into the general direction of where she wants it to happen. She can control the weather. She can move ice around at will, and blast people with frosty gales.
If somebody tries to harm her or her loved ones, she could easily dispatch of them. If a marauding fleet appeared on the horizon, she could freeze them in one spot or crush them between icebergs. Even if an entire enemy army invaded across the mountains, she could just skewer them with icicles or pelt them with 5 cm hailstones until the last one of them stopped twitching, or she could curse their home country with everlasting winter until they sue for peace. Not that any of that would be material for a family movie, but well, that is the extent of her powers as established in Frozen.
So there is a sharply limited number of options for the makers of a sequel:
- Elsa might be only a minor background character. That would most likely disappoint the fans.
- She might not be willing to use her powers offensively against a threat, and the point of the movie might ultimately be how she learns that it is okay to stand up for yourself and your kingdom, and to use force in self-defense. Except for the wee little fact that she has already been shown to be willing to fight back this would make a story - but it does not sound a lot like the take home message the average parents of a six year old would want out of a Disney family movie anyway.
- Her powers could be taken away. Pointless because it would break the character, and thus most likely disappoint the fans.
- She could become evil. Same problem.
- She could be re-traumatized. This is the worst option of all; it would undo all her character development from the original movie and turn the sequel into an uninspired Xerox copy of what we already have. Not that Disney haven't done that before, but it would most likely disappoint the fans.
- Finally, and that seems like the most promising option, it would be possible to have some nefarious plot underway in her kingdom that she is unaware of. The story is about the main protagonist(s) having to gain access to her to present her with proof of the scheme so that she can then effortlessly save the day. Somebody swapping bodies or appearances with or abducting her sister Anna might be a part of it. Although the focus would again be on others, it would make her a powerful presence in the movie while justifying that she doesn't solve the plot with her powers ten seconds after the opening credits.