One interesting concept could be the combination of different game worlds with the Overlord universe. For example, mashing up Overlord with a fantasy RPG world like Skyrim. Characters from Overlord might interact with the unique races and magic systems in Skyrim, creating new and exciting storylines.
Ainz Ooal Gown is often a popular choice. His overpowered nature and complex personality make him a great character to transplant into other game worlds. He can bring his vast magical knowledge and strategic thinking to new settings, which can lead to some epic crossovers.
Cersei Lannister could also be a popular choice. Her cunning and thirst for power in Game of Thrones could be enhanced in the Overlord fanfiction world. She could use her wits and the resources of the Lannisters to become a powerful Overlord - like figure, perhaps even making deals with the dark forces often associated with the Overlord concept.
One great 'Naruto Crossover Overlord' fanfic is 'Naruto in the World of Overlord'. It has an interesting plot where Naruto is transported to the Overlord universe and has to adapt to its rules and power systems. His ninja skills create a unique dynamic in the new world.
One common theme is the exploration of Shalltear's power. Since she's a very powerful character in the Overlord universe, many fanfictions like to test her limits when she's in a crossover situation. Another theme is loyalty. Shalltear's loyalty to Ainz is a big part of her character, and in crossovers, this loyalty often gets challenged or reaffirmed in different ways. For example, in some crossovers, she has to choose between following Ainz's commands and doing what she believes is right in the new world she's in.
Well, there's a 'Overlord Crossover Fanfiction' that combines it with 'Fullmetal Alchemist'. In this, the alchemic concepts of 'Fullmetal Alchemist' are pitted against the magic and power structures in Overlord. It's really cool to see how the characters from both worlds adapt to each other's rules. There's also a crossover with 'Attack on Titan' which brings in the element of the Titans into the world of Overlord, creating a lot of intense and unique battle situations.
One idea could be an Overlord crossover with Sword Art Online. You could have the powerful beings from Overlord enter the virtual world of SAO and cause chaos among the players. They might see the game world as a new territory to conquer or just a place for some interesting 'experiments'.
It's hard to say. Ainz Ooal Gown has a vast array of powerful magic spells at his disposal. However, Ben when he transforms into Alien X could be extremely powerful as well. Alien X has reality - warping abilities. But Ainz has his intelligence and strategic thinking. So it could really go either way depending on the situation.
Power is a common theme. Many fanfictions explore how the characters gain and use power, like in 'Ainz's Ascent' where it shows his journey to greater power.
Character interactions are also unique. For example, Naruto's positive and determined personality clashes and then meshes with the often more complex and morally grey characters in the Overlord world. This can lead to some really interesting character development arcs, like a cynical Overlord character starting to believe in Naruto's ideals of friendship and hard work.