One common theme is adventure. The male reader often goes on adventures with the RWBY characters, which helps to build relationships. Another is the development of the harem. This includes the male reader getting to know each girl in the RWBY group better over time.
One popular 'rwby x male reader harem fanfiction' is 'RWBY: A Male Reader's Adventure'. It has an interesting plot where the male reader character gets involved with the RWBY girls in various exciting adventures. Another good one could be 'Male Reader in RWBY's World: Harem Quest' which focuses on the male reader building relationships with the different RWBY characters in a harem - like setting.
The OC might interact with them in various ways. They could start by being in awe of their combat skills, like Ruby's speed. And in the harem aspect, they could show affection differently to each character. For Weiss, it could be through respecting her family name and helping her with her studies. With Blake, they might bond over books and their love for knowledge.
There are RWBY reader fanfictions that focus on alternate universes. In some of these, the reader might be a Grimm hybrid or have some special powers that change the whole dynamic of the RWBY world. These types of fanfictions are really interesting as they take familiar elements and twist them in unexpected ways.
One great RWBY male reader fanfiction is 'A Male Huntress in RWBY'. It has an interesting plot where the male reader character has unique abilities and has to navigate the world of RWBY. Another good one could be 'RWBY: Male Reader Insert - The New Team'. It focuses on the male reader joining a new team and the adventures that follow. And 'RWBY: Male Reader and the Grimm Slayer' is also worth reading as it involves the male reader in intense battles against Grimm.
One great male reader RWBY fanfiction is 'The Huntsman's Path'. It has an engaging plot where the male reader character gets to interact with the main RWBY characters in unique ways. Another one is 'A New Beginning in Remnant' which focuses on the male reader's journey of becoming a powerful huntsman.
One idea could be that the male reader is a transfer student to Beacon Academy. He has a unique Semblance that catches the attention of the RWBY girls. For example, his Semblance could be related to time manipulation. Ruby might be intrigued because she loves unique abilities and wants to learn from him. Weiss could initially be a bit standoffish but as she sees his skills and determination, she warms up to him. Blake would see something in him that reminds her of her own struggles, and Yang would be attracted to his confidence.
Ruby Rose is a very popular choice. Her speed and scythe - wielding skills make for exciting battle scenes. Also, her leadership qualities can be explored in the context of the battlefield.
Ruby could be a great addition. Her innocence and determination could contrast with the dark and complex relationship between Salem and Ozpin. She might be the catalyst that changes the dynamic between them.
Blake is also a good choice. Her mysterious past and stealthy combat style could add depth to the fanfiction. For example, she could be the one to uncover a hidden plot that Ruby and Scarlet unknowingly stumble into.