One common trope is the 'accidental encounter'. For example, the male reader might accidentally bump into a female character and that starts their initial interaction which could lead to a relationship. Another is the 'protective male' trope, where the male reader has to protect the female characters in his harem from various threats.
One great 'The Owl House x Amphibia' fanfic could be one where the characters from both shows team up to face a common threat. For example, Luz from 'The Owl House' and Anne from 'Amphibia' might combine their magical and survival skills respectively to defeat an evil force that's trying to take over both their worlds.
Well, a major theme is the power of attraction. The male reader usually has some qualities that attract the female characters to him, like kindness, courage, or a special ability. Another theme is the exploration of different personalities. Each female in the harem has her own distinct personality, and the male reader has to understand and deal with them all. And, of course, there's the theme of love and loyalty. The relationships in the harem are often based on some level of love and loyalty among the characters.
Romance and relationship building is another big theme. The male reader has to figure out how to balance his feelings for multiple characters. This can lead to a lot of drama and tender moments. Like in some fanfictions, there are love triangles or even more complex relationship webs within the harem.
One common theme is self - discovery. The male reader character often discovers new aspects of himself while interacting with the harem members. Another is power growth. As he forms relationships, he may gain more power or influence. And of course, romance is a big theme as he develops feelings for the different female characters in the harem.
One common theme is the male reader's charm or special abilities that attract the female characters. For example, he might be super intelligent or have some kind of hidden power.
One popular harem x male reader fanfic is 'My Harem in Another World'. It has an interesting plot where the male reader is transported to a different world and gradually forms a harem. The characters are well - developed and the story has a good mix of adventure and romance.
There's a fanfic titled 'Hunter and Willow's Adventure'. In this, the author takes them on a journey across different parts of the Isles that are not shown much in the original series. It's full of action and also some sweet moments between the two characters. The descriptions of the settings are really vivid which makes it an engaging read.
Another place to check could be FanFiction.net. Although it has a different layout and community compared to AO3, it still hosts a lot of fanfics. However, be aware that the quality and content can vary widely. You might need to sift through a few stories to find the ones that really interest you regarding the 'The Owl House x Amphibia' crossover.