An enchantress could also be a love interest. She might offer Edward a solution to his heir problem through magic, but in return, she wants something from him. Their relationship could be a complex one of power, love, and mystery. Edward would have to navigate through her world of magic while also dealing with his own desires for an heir and the future of his kingdom.
There could also be some original characters created just for the fanfiction. Maybe a new female alchemist who crosses paths with Edward and they end up having a relationship that leads to him becoming a baby daddy. Another possibility is an old acquaintance from his travels who resurfaces and they develop feelings for each other, resulting in a child.
One idea could be that Edward embarks on a journey across different kingdoms to find a suitable partner to have an heir. He might encounter various challenges and meet interesting characters along the way.
Perhaps a mortal with a special gift for metallurgy or some other craft related to Hephaestus' work. This mortal could be seen as an equal in terms of skill, at least in a way that catches Hephaestus' eye. They would bond over their shared love for the craft, and over time, this bond could turn into a deep, loving relationship. Hephaestus might be drawn to the mortal's unique perspective and the way they approach the craft despite not having the same divine powers as he does.
Edward could be an ordinary man living in a small town. He has always wanted a family of his own but couldn't find the right partner. So, he decides to adopt a child to start his own family unit.
It could be a rival vampire clan. They might see the son as a threat because of his unique nature, being part human and part vampire. They may think that as he grows up, he could become very powerful and pose a danger to their own power and territory in the vampire world.
Hinata is a very obvious choice. She has always had deep feelings for Naruto. In a fanfiction, their relationship could spiral further, exploring more of their shared interests, like their love for the village and their dreams for the future of the ninja world.
A mysterious traveler passing through the town could be a new character. This person has seen many strange things in their travels and is not afraid of Edward. In fact, they see his potential and encourage him to leave the town and see the world.
If it's a crossover with the Percy Jackson universe, Percy himself could be interested in Ruby. They both have a sense of adventure and a desire to protect their friends. Annabeth might find herself drawn to Weiss because of their shared intelligence and strategic thinking. Leo Valdez could have a crush on Yang due to her mechanical skills and fiery personality. And Nico di Angelo might be intrigued by Blake because of her mysterious nature. These could all be great potential love interests in a RWBY - Percy Jackson crossover harem fanfiction.
Ezra might also be a possibility. He could be involved in a complex relationship with Spencer where he has to deal with her pregnancy and all the emotions that come with it.
There could be an older, more mature character, like a college student who she meets by chance and who is drawn to her unique charm and innocence.