Voldemort could be one of the main antagonists. He could follow the female Harry Potter through the portal to Middle - earth, causing chaos there. Also, the orcs and other dark creatures from Middle - earth like the Nazgûl could be antagonists. They would oppose the female Harry Potter and the hobbits or dwarves she teams up with.
One idea could be that a female Harry Potter somehow gets transported to Middle - earth from the wizarding world. She could use her magic skills to help the hobbits in their quest. Maybe she teaches them some simple spells for protection or to make their journey easier.
One key element is the character of fem Harry Potter herself. Her different personality traits compared to the original male Harry Potter can greatly influence the story. For example, she might approach challenges in a more empathetic way. Another element is the merging of the two worlds. The magic of Harry Potter's world and the unique races and cultures in The Hobbit's Middle - earth need to be blended well. This could include how spells interact with the creatures like dwarves and hobbits.
A popular one is 'Fem Harry and the World of Middle - earth'. This fanfic is really interesting because it crosses over the Harry Potter universe with Tolkien's Middle - earth. Here, Fem Harry has to adapt to the very different cultures and magic systems in Middle - earth, and there are some great character interactions that add depth to the story.
One interesting plot could be that the wizards from Harry Potter world accidentally stumble upon the Shire while on a magical experiment gone wrong. They meet the hobbits and learn about their simple yet fulfilling lives. Meanwhile, the hobbits are in awe of the magic the wizards possess. Then, an ancient evil that threatens both the wizarding world and Middle - earth emerges, forcing them to combine their strengths. The wizards use their powerful spells and the hobbits' courage and resourcefulness to fight against it.
One popular fem Harry Potter Hobbit fanfiction is 'The Hobbit in Hogwarts'. It combines the magical world of Harry Potter with the charm of Middle - earth from The Hobbit. In this fic, a female version of Harry Potter somehow ends up in Middle - earth and has to use her Hogwarts - learned magic to survive and make her way back home.
Another great option for beginners is 'Fem Harry's First Adventure in Narnia'. This fanfic doesn't go too deep into complex crossovers. It focuses on Fem Harry's journey into the magical land of Narnia. The story is written in a very accessible way, with clear descriptions of both the Harry Potter - like elements and the new Narnia world she discovers.
One idea could be that the Hogwarts students go on an exchange program to Middle - earth. They could learn magic from the wizards there like Gandalf and in return teach them some of their own spells. Another could be that the One Ring somehow finds its way to Hogwarts and causes chaos among the students and teachers as they try to resist its power. Or perhaps a character like Harry Potter is transported to Middle - earth and has to use his skills to help the Fellowship in their quest.
The theme of identity is also prevalent. The fem Harry Potter has to figure out who she is in a world that has certain expectations. With the crossovers, there are additional factors that influence her identity. She might have to balance her Hogwarts self with the new elements introduced from the other fictional universes she is crossed over with.
One great fem Harry Potter crossover Naruto fanfiction is 'The Potter-Namikaze'. It combines the magic of the Harry Potter world with the ninja skills of Naruto. The story follows a female Harry Potter who somehow ends up in the Naruto universe and has to adapt. It's full of interesting character interactions and unique plot twists.