If you were going to tell a HP story using the Buffy formula with a season-arc supernatural "big bad", who would be your big bads?

Some ideas:

- A Fiendfyre-based entity which gains full sentience and seeks to consume the world in fire.

- A sentient Potion which takes over the bodies of the people who drink it, compelling them to make more of the Potion and sneak it into other people's drink.

- A shade of the dead, summoned with the Resurrection Stone, which managed to overpower its summoner, take possession of the Resurrection Stone, and now seeks to turn the living world into a world of the dead.

- A squib who is the sole heir of a prestigious pure-blood family. He has found an ancient dark ritual whereby he can ascend to wizardhood through a series of ritual sacrifices.

 

-An apprentice wandmaker has gone missing. Turns out a rogue goblin faction has kidnapped him, and is extracting the secret of wandmaking, in an attempt to then share it with the wider goblin population and put wizards and goblins on equal footing.

Optionally, can be secretly a gringotts funded operation, disguised as an extremist faction for plausible deniability.

-A rogue unspeakable has stolen an experimental time turner that lets you go back years, and is trying to change history, potentially for a 'good' reason, like say assassinating Hitler or Grindelwald.

-For nearly every spell or charm, there is a possibility to create a better, more effective spell with research and experimentation, like say a basic levitation charm and the locomotor charm.

An underground wizard research cell is researching more advanced versions of the unforgivables.

 

I would be incredibly interested in reading a story where 'canon' is the offshoot of a timetraveller, and they're now trying to prevent a timetraveller from the real future destroying their own.

It ventures into the same waters as FGO Pt 2, but told from the other side.



- Merpeople are the major investors in Big Oil/Coal firms, and are deliberately trying to melt the ice-caps to gain access to more of the planet.

- A 'muggleborn' auror is actually descended from a squib line, and is both murdering, and framing, the wizarding descendants of his great-grandfather in a tumultuous summer of backstabbery so that he can emerge afterwards and lay claim to the manor house. The other murders he does on the side are just a smoke-screen.

- Fenrir Greyback is attempting to turn members of the British Royal Family

 

A werewolf serial killer (Greyback or otherwise) would have to be a consideration for that sort of big bad.

A goblin gangster, running a shady underground organised crime network across Diagon and Knockturn.

A coven of hags who kidnap and sacrifice (eat?) babies.

A ghost thats found a way to temporarily gain solid form - by syphoning the life out of nearby mortals.

A paranoid schizophrenic metamorphmagus whose deteriorating mental state and ability to completely disguise themselves result in unpleasant consequences.

 

- A band of goblins become disillusioned with the work required to extract gold and silver from the earth and instead they figure out a great way to melt down humans they kidnap and salvage the milligrams of precious metals they contain

- An unusually bright dementor that learns to brew love potions to lure more victims to itself

- The godess Hecate (or some other ancient god of magic) decides that she doesn't like for humans to use magic anymore and puts a curse on human magic. Every time a witch or wizard performs magic, someone else loses their magical power and becomes a squib. It takes days for the wizarding orld to figure out what's happening and soon only one human with magical power remains...

 

Students at Hogwarts begin to go missing, but there is always an explanation - a girl's family has taken her on holiday, a boy transferred schools because he's been failing exams, someone else was made an apprentice. Turns out a group of extremist centaurs in the forbidden forest were kidnapping outcasts for a ritual to increase Mars's brightness.

 

I had one idea in this vein recently.

One of the seventh year Ravenclaw prefects has been attending the school for almost a century. At the start of his final year at Hogwarts, he conducted an experiment to try and infuse a Time Turner's power into himself so he could time travel at will, but the attempt backfired, trapping him in a localized time loop. What's worse, even as he absorbed the Time Turner, a part of himself has seemingly been assimilated into the school, preventing him from leaving the school grounds or making his plight known to others. Now he's stuck repeating his seventh year over and over, even as the students and staff around him continue their lives as normal, seemingly oblivious to the unwilling immortal in their midst.

And so with no other options and nothing else available to stave off a descent into madness, the Ravenclaw spends his time plumbing the depths of the castle's mysteries, as well as researching as much experimental magic as he can in the hopes of breaking the cycle. Nothing he comes across seems to help - until one year, when a certain precocious third year comes to Hogwarts, having been gifted a Time Turner of her own.

Probably a bit convoluted, but there you go.