Maybe the antagonist is a group of Ravenclaw alumni who are part of a secret society. They are using Ravenclaw's reputation for knowledge to hide their dark deeds. Harry discovers their plot and decides to disown Ravenclaw publicly to expose them. These alumni would then try to silence Harry and protect their secrets, making them the main antagonists in the fanfic.
It could be a Ravenclaw student who has been misusing their intelligence. They might be creating chaos in the wizarding world, and Harry's disowning of Ravenclaw could be his way of standing up against this misuse. This student could be jealous of Harry's fame and try to undermine him by causing problems that would make Harry take such a drastic step.
The main antagonist could be an old Ravenclaw ghost or spirit who has been influencing the current Ravenclaw students to follow a more self - centered and power - hungry path. This spirit may have some old grudges or a thirst for control, and Harry's action of disowning Ravenclaw is his attempt to break free from this negative influence. It would be a battle between Harry's sense of right and wrong and this ancient, malevolent force.