There could be a new villain in the form of a time - traveling dark wizard from the past. He has come to the present day to change the future in his favor. He sees Hogwarts as a key institution that can be manipulated to achieve his goals. He starts by infiltrating the school as a new teacher. He then begins to subtly influence the students, turning them against each other and weakening the school's unity from within.
It could be a powerful veela who has a grudge against Hogwarts. Maybe a Hogwarts student in the past wronged her family. She uses her charm and powerful magic to try and bring chaos to the school. She first lures some of the male students with her beauty and then uses them to carry out her devious plans. For example, she might make them steal important magical artifacts from the school that are used to keep Hogwarts safe.
One possible plot could be that Hogwarts is facing a new and powerful dark magic threat. Maybe a long - lost dark wizard's spell has been activated and is slowly draining the magic from the school. The students and teachers then have to work together to find a way to reverse the spell. They might search through old tomes in the restricted section of the library and go on adventures outside the school to gather rare ingredients for a counter - spell.
It could be a rogue Death Eater who escaped punishment and has been biding his time. He might have a new, more insidious plan to bring back the dark ways of Voldemort.
Some dark wizards who were in hiding during the main battle might emerge. They could see the post - battle chaos as an opportunity to gain power.
Another possibility is McGonagall again. She has been an excellent teacher and deputy headmistress for a long time. Her experience and strict but fair nature would ensure the smooth running of Hogwarts. She already has a great relationship with the students and faculty, and could continue to uphold the school's traditions while also making some positive changes.
It could be a long - lost relative of Voldemort, carrying on his dark legacy.
A possible main villain could be an ancient sorcerer from the new dimension who has been awakened from a long slumber. He's enraged that the wizarding world has been interfering with his dimension's magic, and decides to take revenge on Harry Potter's world. He has powers that are unknown to the normal wizarding world, like the ability to control time in his dimension, which makes him a formidable foe.
Another possible antagonist could be a power - hungry pure - blood family. They might see Harry as an impostor as the heir, believing that only a pure - blood should hold such a title. They would try to undermine Harry at every turn, spreading rumors and creating obstacles for him both in school and in the wizarding world.
It could be a new dark wizard who has infiltrated Hogwarts under the guise of a new teacher. This wizard might have a vendetta against Harry specifically and sets up elaborate traps to kill him.
His friends for sure. Hermione is really smart and could help with things like making sure the baby is healthy with magic, and also helping with any magical problems that might come up because of the baby.
Luna Lovegood's father, Xenophilius Lovegood. He's an eccentric but knowledgeable wizard. He could introduce Harry to some very unique forms of magic and different ways of thinking about the magical world. Harry would get to explore some really out - there magical concepts under his mentorship.