One plot twist could be that the dragons are the key to saving Arendelle from a new, more powerful form of Elsa's ice magic that has gone out of control. This forces the characters from both universes to re - evaluate their views on the dragons and work together in ways they hadn't before. It's a unique take on the combination of the two worlds and their respective powers.
A really interesting plot twist is when the villain in the fanfiction turns out to be a character who was initially thought to be an ally. For example, in a story where the Vikings and Arendelle are working together, a seemingly helpful Arendelle noble is actually in league with a dragon - hunting group from httyd. It throws everyone into chaos and makes for a great read.
One interesting plot twist could be that Percy's water powers somehow interact with Elsa's ice powers in unexpected ways. Maybe when they meet, their powers combine to create a new form of frozen water element that has unique properties. Another could be that the demigods from Percy Jackson's world have to deal with the magical cold of Arendelle, which is a whole new kind of challenge compared to their usual Greek - related threats.
One idea could be that the dragons in HTTYD are attracted to the cold of Frozen's world and they fly there. The people of Arendelle are initially scared but then they learn about the dragons from the dragon riders. Another idea is that Elsa's ice powers somehow have an effect on the dragons, like making their fire breath turn into ice breath for a short time.
In some fanfictions, like 'The Unlikely Alliance', the plot twist is that the Hulk is actually being controlled by an unknown villain who wants to use him to take down Batman. Batman has to figure out not only how to stop the Hulk but also who is behind it all.
One interesting plot twist could be when a Splatoon character suddenly gains Naruto - like powers. Imagine an Inkling being able to use the Rasengan. It completely changes the dynamic of the story.
A really interesting plot twist can be when the characters from Undertale end up in the Naruto world and they have to adapt to the ninja way. For example, Sans using his shortcuts in the middle of a ninja battle, which throws everyone off guard. It's unexpected and adds a whole new level of humor and chaos to the story.
Some fanfictions have a plot twist where a minor character from the original Battle of Hogwarts turns out to be a key figure in saving the day. For example, a previously unnoticed student might have a hidden power that changes the outcome of the battle.
In some fanfictions, a big plot twist is when Naruto suddenly gains a Devil Fruit ability, completely changing the power dynamics. For example, in 'Naruto's Devil Fruit Adventure', he gets a powerful fruit that makes his ninja skills even more formidable.
One really cool plot twist is when the values of the Naruto universe start to influence the Decepticons in unexpected ways. For example, the concept of friendship and loyalty that Naruto and his friends uphold might start to make some Decepticons question their own motives and allegiances, which leads to a whole new set of conflicts and resolutions in the fanfiction story. This not only adds depth to the story but also makes for a very engaging read as it challenges the typical Decepticon - like behavior we are used to in the original Transformers franchise.
Perhaps a plot twist where an evil force is manipulating both their universes and they are the only ones who can see the signs. Naruto and Caulifla must put aside their differences and combine their powers in a way no one thought possible to stop this greater evil that threatens both their worlds and the fabric of reality itself.
Another interesting plot twist is when the main villain in the Naruto world turns out to be in cahoots with an evil Bakugan from another dimension. This forces Naruto to not only fight his regular enemies but also understand the complex politics and power struggles in the Bakugan world to stop the greater threat. It's a really cool way to merge the two universes.