One common theme is the clash of cultures. Since The Last Kingdom focuses on Saxons and Vikings focuses on, well, Vikings, when they cross over, there's always a lot of exploration of how these different cultures interact. Another theme is the idea of loyalty. Characters from both shows have strong loyalties to their own groups, and in the crossovers, this loyalty is often tested.
There's also 'A Meeting of Two Worlds' which is a well - written fanfic. It delves into how the cultures from both shows would interact. The author does a great job of keeping the characters in - character and creating a believable story where the two universes collide. It has some exciting battle scenes and political intrigue that fans of both shows would enjoy.
A crossover could start with a portal opening between the two worlds. A group of students from Hogwarts, including Harry Potter himself, accidentally step through it and end up in The Last Kingdom. They are shocked by the primitive living conditions compared to their magical world. But they also notice that there are some hidden magical elements in this world that are different from what they know. They then decide to use their magic to improve the situation in The Last Kingdom, for example, by using spells to heal the sick or protect the villages from invaders. And in the process, they form alliances with the locals and have many adventures.
A cool vikings crossover fanfiction idea is to cross it with 'The Witcher'. Vikings could meet Geralt and get involved in his monster - hunting quests. They might see the magic and creatures in the Witcher world as either a threat or an opportunity. Also, a crossover with 'Assassin's Creed' could be interesting. Vikings could encounter the Assassins and Templars, and their own beliefs and values could clash or align with these secret organizations.
Definitely Harry Potter could be one of the main characters. He has the courage and magic skills to be a key figure in the crossover. Another could be Uhtred from The Last Kingdom. His warrior skills and knowledge of the land could pair well with Harry's magic. Hermione might also be a main character as she could use her intelligence to figure out how the two worlds interact.
Thor would likely be seen as a very powerful and strange figure by the Vikings. They might be in awe of his strength and his magic hammer. Some Vikings might be very eager to prove themselves to him, while others could be more cautious. Thor, on the other hand, might view the Vikings as brave but primitive at first. He could gradually come to respect their fighting spirit and honor code.
Ragnar Lothbrok is a very popular character to feature. His charisma and leadership make him a great fit for crossovers. He could easily hold his own against characters from other universes. Lagertha is also popular. Her strength as a female warrior is appealing, and she could have interesting interactions with other strong female characters from different stories. Floki, with his unique personality and skills, is another one. His unpredictable nature can add a lot of drama to a crossover story.
One plot idea could be Thor landing in the Viking era and being mistaken for a god. The Vikings might try to either worship or challenge him. Another could be a Viking warrior being chosen by Thor to help him on a quest in Asgard. Or perhaps Thor has to team up with a group of Vikings to fight a common enemy like a giant frost monster.
One idea could be a scenario where the Vikings' raiding parties encounter the Night's Watch from Game of Thrones. Maybe they initially see each other as threats, but then realize they could be useful allies against the White Walkers. Another could be a story about a character from Vikings, like Ragnar Lothbrok, being magically transported to Westeros and having to navigate the complex politics there, similar to how he did in his own land.
A Vikings - Game of Thrones crossover fanfiction could feature epic battles between the Vikings' fierce warriors and the various houses in Westeros. Maybe Ragnar Lothbrok could meet Jon Snow, and they would exchange their combat and survival skills. It could also explore how the different cultures and belief systems interact, like the Vikings' Norse mythology and the religions in Westeros.