One possible motive could be ransom. Maybe some criminals knew that Christian was wealthy and thought they could get a large sum of money for his and Ana's release. Another motive could be revenge. Perhaps someone who had a grudge against Christian or Ana from their past decided to take this extreme measure.
The kidnappers might be part of a cult or a fanatical group. They could believe that Christian and Ana were symbols of something they opposed. So they kidnapped them to make a statement or to try and convert them to their way of thinking. Maybe they thought that Christian and Ana's relationship went against their beliefs and they wanted to 'correct' it.
It could be that the kidnappers were after some valuable information that they thought Christian or Ana possessed. For example, if Christian was involved in some important business deals, the kidnappers might want to force him to give up trade secrets. Or it could be related to Ana's family secrets that someone was eager to uncover through kidnapping them.
Christian and Ana were enjoying a day out when they were kidnapped. The kidnappers had mistaken them for someone else. They were tied up in a dark room. Christian tried to communicate with Ana to keep her spirits up. He noticed that the ropes binding them were a bit loose. He worked on loosening them more and finally freed them both. Then they carefully made their way out of the kidnappers' hideout.
The motives could be revenge. Somebody who has been wronged by Deidara or the Akatsuki in general might kidnap him to get even. For example, a village that was destroyed by his explosives could seek vengeance.
It could also be for power. Some evil shinobi might believe that by capturing Naruto and somehow extracting the Nine - Tails' power, they can become extremely powerful. They might have some sort of dark jutsu or experiment in mind that requires the power of the Nine - Tails which is sealed within Naruto.
Another motive could be revenge. Maybe some in Iwa still hold grudges against Konoha for past conflicts. Since Naruto is a prominent figure in Konoha, kidnapping him would be a way to strike back at the village. They might think that by harming Naruto, they can make Konoha suffer.
Perhaps in this fanfiction, Dumbledore has a secret plan related to Harry's power. He might think that by kidnapping Harry, he can train him in isolation to fully develop his magical abilities without the distractions and potential dangers at Hogwarts. It could also be that Dumbledore is trying to hide Harry from Voldemort in a way that no one would suspect, as Voldemort would never think Dumbledore would use such extreme measures against Harry.
Perhaps in this fanfiction, Christian kidnaps Ana thinking he's protecting her from some unseen danger. He might believe that the world outside is too dangerous for her and this is his way of keeping her safe, although it's a very extreme way.
Well, Obito could be doing it out of revenge. He has a long - standing grudge against the Hidden Leaf Village. Naruto is seen as a symbol of hope in the village. So, by kidnapping him, Obito could be trying to strike a major blow to the village's morale and show that their so - called 'hope' can be easily taken away.
One possible motive could be to gain power. Since Goten is a Saiyan, his power could be harnessed in some way. For example, they might want to extract his ki or use his DNA for some evil experiment.
Perhaps they meet in a coffee shop. Ana is sitting there, reading a book, and Christian comes in by chance. He accidentally bumps into her table, and as they exchange apologies, they start a conversation that leads to more.
Maybe Saoirse Ronan for Ana. She is a very talented actress who can convey a lot of emotion. And for Christian, Robert Pattinson could be a possibility. He has experience in playing complex and somewhat mysterious characters.