Long after the wizarding world had fallen, the walls of Hogwarts remained...
None now know for certain what terrible magic it was that cracked the cradle of civilisation and laid waste to the Wizarding World. The knowledge of great sorcery passed out of human knowledge along with its wielders, at the end of the last great war. Now, all that remains of the Kingdom of Great Britain is a handful of isolated baronies, separated by miles upon miles of haunted wastelands where strange monsters dwell.
It is said that Hogwarts School of Witchcraft and Wizardry played an important role in the last war, though the specifics are unclear. Regardless, the school now lies abandoned under an evil curse, and is inhabited by all manner of evil creatures. Yet it is also rumoured to contain a great deal of treasure left by the wizards of old who were wiped out in an arcane cataclysm. Many brave or foolhardy souls have sought this treasure in the castle's benighted halls, but few have returned.
For those adventurers who prefer fresh air and daylight, the nearby Forbidden Forest also offers opportunities. Inhabited by centaurs, pixies and giant spiders, the forest stretches north an uncharted distance into the far reaches of Scotland. Somewhere on the far side of the forest, so it is said, lies the Kingdom of Giants which was, perhaps, untouched by the final war.
There are other places, also, where the honest and steadfast would never dare to go. Tales abound of the island prison of Azkaban, somewhere in the North Sea, where the most vile criminals were trapped in prisons of madness. But now, they say, the prisoners have escaped and taken up rulership of the isle, making themselves evil masters of an insane republic. All know of the Lich Lord in the west, who rules immortal from a castle of black stone. And it is whispered that the goblins of Gringotts, once thought perished in the conflagration, have survived and relocated their gold stores to a new vault deep beneath a northern mountain.
South of the Highlands the land grows increasingly bleak and twisted, and no travellers can survive in the ruinous country that was once known as 'England'. To travel all the way to fabled London, and delve the sub-basements of the legendary Ministry of Magic building at Whitehall, is a dream that even the mightiest adventurers hardly dare hope for. Yet it seems that something has crawled out of the nuclear carnage of the southlands: for recently on certain way-signs and tree trunks have appeared mysterious graven letters spelling out 'D.O.M.'
This feels like a version of Hogwarts that I would be more excited about running. The high school drama of my first concept would be superficially amusing, but in the long run I think this version would give the campaign more space to grow outside of the Hogwarts dungeon. I could see this growing to a domain level game where the PCs either renovate Hogwarts or build their own castle/s somewhere else, getting involve in Lyonesse-style politics with the local lords and other factions like the Republic of Azkaban.