A powerful foreign dark wizard could be the main villain. He might have heard of the prophecy about Harry Potter and decided to steal him from the Dursleys. This wizard could come from a far - off land with different and more dangerous dark magic, and his motives could be to gain ultimate power in both his own land and the British wizarding world by controlling Harry.
Maybe an evil house - elf. House - elves are often underestimated in the wizarding world. An evil one could have stolen Harry as a baby with the intention of using his magic to break the bonds of servitude that all house - elves have. He could keep Harry in a hidden place and try to force Harry to use his magic for this purpose.
It could be a disgruntled house - elf. Maybe one that was mistreated by its wizarding family and decided to steal money as revenge. They could use their magic to get into places unnoticed and take the money.
One possible plot could be that a powerful dark wizard, other than Voldemort, steals baby Harry for his own dark purposes. Maybe he wants to raise Harry to be his apprentice in the dark arts. He could take Harry to a hidden lair far away from the wizarding world, and as Harry grows up, he starts to suspect that his 'guardian' is not a good person.
Well, Harry is the obvious main character. He has to deal with the situation of his stolen inheritance. Then there could be a new, mysterious character who is the mastermind behind the theft. This character might have some hidden agenda against Harry or the Potter family. Ron and Hermione would be important secondary characters, providing support to Harry in his quest to reclaim what's his. There could also be some minor characters from the original series who get involved in the story in unexpected ways.
Maybe there are some new characters introduced in this fanfiction. It could be a young wizard who discovers the stolen magic first and then teams up with the well - known characters. Or it could be an old wizard who has some knowledge about this kind of magic theft and guides the others. However, it's also possible that the focus is still on the core trio, and they are the ones facing the challenges related to the stolen magic.
It could be that the main character is an original creation for this fanfiction. However, it's very likely that some of the well - known characters from the Harry Potter series play important roles as well. For instance, if the story is about a child's difficult upbringing in the wizarding world, Dumbledore might be involved as a guiding figure. Or perhaps Draco Malfoy could be part of the story, showing how his own childhood was also influenced by his family's actions and beliefs, which in turn affects the main character's 'stolen childhood' story.
Maybe the baby is a result of some sort of magical experiment gone wrong. And Harry Potter finds it in a hidden place where the experiment was carried out. The baby might have some strange and unpredictable magical abilities as a consequence.
It could be a new dark wizard. Maybe a jealous former Hogwarts student who felt overshadowed by Dumbledore and now sees baby Harry and Hermione as threats to his future plans. He might want to use their power for himself or just eliminate them to make sure he can rise to power unopposed.
It could be a new character created by the fanfiction author. Maybe a powerful wizard who was once shunned by the wizarding community and now wants revenge by stealing magic.
A dark wizard who has merged the dark arts with Wonderland's magic could be the main villain. He might have created some sort of dark Wonderland where all the creatures are under his control, and Harry has to break this control to restore balance.
Ron and Hermione could be main characters. They are Harry and Ginny's closest friends. Hermione might offer her vast knowledge of magic to help with any problems the baby has, like if there are some strange magical symptoms. Ron could be there for comical relief, making jokes about how Harry has become a proper family man now.