It could be Draco Malfoy. He might make fun of Harry for his new fitness obsession, thinking it's a silly Muggle - like pursuit. And this could lead to a series of confrontations between them.
The Dursleys could be the antagonists in a way. They might find out about Harry's new fitness regime when he's at home during the holidays and try to stop him, thinking it's a waste of time or something that makes him different from their idea of a 'normal' boy. They could even try to sabotage his efforts by not allowing him enough food or privacy to exercise.
In this fanfiction, perhaps Harry Potter decides to get in shape for some new challenges at Hogwarts. He might start a strict exercise routine in the Room of Requirement. He could be motivated by wanting to be stronger for the upcoming battles against dark forces or to impress his friends.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
In this fanfiction, it might be that the main antagonist is actually a part of Harry himself. His inner demons, so to speak. The 'wasted' state could bring out his insecurities and negative traits, and these could manifest as an internal struggle. So, in a way, Harry is his own antagonist as he has to overcome this self - destructive side of him.
It could be a rogue wizard who is jealous of Harry's new cybernetic abilities. This wizard might think that Harry is polluting the purity of magic with technology and decides to stop him at all costs.
It could be Voldemort still. Even with Harry having the powers of the Sage of Six Paths, Voldemort would be a formidable foe. He might be intrigued by this new power and try to steal it for himself.
The main antagonist could be the Ministry of Magic if they view the succubus as a dangerous and uncontrollable creature. They might try to separate Harry from his familiar, and in the process, become the main force that Harry has to oppose.
Voldemort could still be the main antagonist. Even with a new wand, Harry is still a threat to him, and Voldemort would be eager to get his hands on such a unique and powerful wand.
Yes. In the vast world of fanfiction, the idea of Harry Potter having a six - pack isn't too far - fetched. Fanfiction allows for a great deal of creativity. Writers may have been inspired by modern ideas of fitness and attractiveness and decided to incorporate it into their version of the Harry Potter universe. This could also be part of a plot where Harry is using his physical appearance in some way, perhaps to charm or deceive in a story that has a more complex or different take on the magical world.
It could be Draco Malfoy. Maybe he somehow manages to use a new spell or a dark object to turn Harry into a ferret out of spite. He might then try to keep Harry in that form to humiliate him in front of the whole school.
A lot of fanfictions focus on different aspects of the characters. While I can't point to one particular six - pack fanfiction, it's possible that some stories that center around a more grown - up and physically - fit version of the characters after the Battle of Hogwarts might include this aspect. You could also check out forums where 'Harry Potter' fans discuss their favorite fanfictions. They might be able to recommend some good ones with this theme.