Another theme could be redemption. Writers might explore the idea of the reader character being the one to bring out a different, more positive side of Hans Landa. There can also be themes of love against all odds, as Landa is a morally ambiguous character, and the relationship between him and the reader can be a very difficult one to navigate in the context of the story.
A common theme is the exploration of the complex relationship between the reader and Hans Landa. Given his character's nature in the original work, there's often a sense of tension and intrigue.
Power dynamics also play a significant role. Landa has a certain amount of power in the original setting, and in fanfiction, the exploration of how that power affects the relationship with the reader is quite common. Whether it's the reader being in a submissive or dominant position in relation to Landa's power, or them trying to break free from it, these are all elements that can be found in many hans landa x reader fanfictions.