So I want to go at this pretty much from memory. I could pull the books down or more reasonably/ go out and check the various interwebs to get all of the details. But then I might find out I'm wrong about something. And I don't really want to sweat the fine crunchiness of this too much- instead I'm working on impressions. I'd had a particular campaign hack in my head for some time. Then a thread with a great related idea popped up on RPG Geek. That's worth reading. In a nutshell it proposes a Call of Cthulhu-like campaign with players stumbling into a world of dark magic, and then eventually finding their way to Hogwarts-- having been led there by a trail of the slain left by the Death Eater factions and the big war. Voldemort and his crew look like evil wizards to the muggles, so why shouldn't there be a few crusading muggle investigators just now putting the pieces together.
Related: My earlier discussion on running school-centered campaigns.
I love hacks which take an existing idea and consider what it would look like to an outsider or a newcomer. I used the rabid craziness of the Unknown Armies setting not as the primary place for the PCs, but as something they came into contact with and gradually learned about. The Promethean line from the new World of Darkness seems more than a little unplayable as a campaign. However those ideas make for great NPCs. In a Vampire campaign I used a classic Hunter the Reckoning character with all of the classic social/psychological problems given by their origin as a foil. It took them some time to figure out exactly what was happening with that character. In the cases I'm citing it works in great part because most of my players haven't read that material, so the revelation and pulling back of the curtain works as a nice surprise. I wonder how long you could sustain that uncertainty before you'd reveal the Harry Potter connection in the game?
There are some other great suggestions in that thread as well. Let me begin with a few things drawn from my recollection of the Harry Potter books. A set of details and premises, if you will.
*The history of wizards certainly stretches back some time. We have a catalog of history in the form of the book of somewhat Medieval tales handed off to Hermione, in the last book. I like to think of those as a childrens' version of the Canterbury Tales for Wizards.
*We have a lengthy history of wizards hidden from the Muggle (mundane) world.
*We have a continuous history of some families and lines involved with the practice.
*We have several major sites for these schools- Hogwarts in England, plus the clearly French one and the German/Russian one.
*We have a founding of the Hogwarts school coming from the gathering of several like-minded wizards who came together- Gryffendor, Slythern, Hufflepuff, etc.
*Wizards use a faux Latin to cast their spells.
So there's a particular reading on the history that has Ars Magica as the precursor to the modern magic of Harry Potter.
Hogwarts (and the other two schools) end up as particularly focused and specialized covenants. One can imagine that places like the Ministry of Magic HQ and the like grew from those as well. As the world in general moves to higher levels of communication and specialization, and out of the decentralized and feudal systems, the Covenants of Ars Magica would have to make like changes. One of the most important, of course, being the break down of the classic Apprentice/Master/Guild system. However I imagine that probably hung around for a good long time. Perhaps Hogwarts and the others went towards a radical practice of training wizards in the rudiments before sending them out to be more formally apprenticed. That would explain the evolution and difference.
FAQ (Not Really...)
There are a couple of other things which need to be answered in this frame:
*What about Vis and the energy requirements for some magics? Wands easily answer this question. At some point wizards managed to discover some kind of energy system which could be tapped into. Specialized devices were required leading to the creation of wands. Some resented that development, regarding them as a crutch. Eventually they wold become so integral to casting that most would regard them as an essential tool.
*Wizardy's at least partially genetic in the Harry Potter books? Which remains one of the things I kind of dislike about them. There's some of the midichlorian problem in this. However HP does assume that the talent does appear in Muggles, or Mudbloods as they call them. So lets assume that there's some factor for the inheritance of magical talent. A discovery of that nature might have made for some significant inbreeding or arranged marriages within the Wizard community. Certainly there's the suggestion of that being a problem by the time of the HP books-- that some of the most potent families fell because of that kind of limited genetic pool. I think in general this could be a non-issue unless you wanted to make an arranged marriage a plot device in a campaign.
*But how do you get the spell system to work? By the modern era we've got mages existing in a bureaucracy. The focus has become working with existing techniques and mastering those. We don't have any particular sense of the wizards creating new kinds of magic-- except for very old and wizened characters. I'd argue that the old system of spells got transformed into a more literal shorthand-- which in turn became the key and accepted form of magic. Essentially wizardry in the Harry Potter universe of modern-day is a more ossified version of the craft. Because they cast magic based on a particular kind of power source and approach, they've limited their powers.
*What about Twilight, the Seasons, and all of that Ars Magica chrome? If you're running a historical game, then these elements could be applied. However it could be easily argued that developments of larger and tighter multi-Covenant organizations would be able to keep down instances of Covenants falling into disrepair and decay. At the same time new developments in the thinking about therapies and treatment might allow the wizards to be more proactive and efficacious in dealing with those who might be going off the deep end.
OK, what can we build from this? A couple of campaign frames suggest themselves
*The Founding of Hogwarts-- not the literal one, but perhaps the generation after Ravenclaw and the others founded it. The place still looks a lot like a Covenant. But there's a slightly different approach in that they're not focused on research, but on teaching a larger number of students within each house. Perhaps the PCs start as students and then become instructors. You'd have suspicion and opposition from other places-- bringing in the conventional Ars Magica Houses (Flambeau, Ex Miscellania) as details. One could weave those threads together. You could also tell stories about how the grounds got established and the like. Perhaps there could be some interesting focus on how other traditions became integrated into the larger world of wizardry. I love those kinds of transitional stories and you could really mash the two together. I'd assume this would be set sometime in the Early Modern Period, perhaps even the English Civil War or after. There's a great potential mash up if we have the Musketeer period and details as well. Depending on your historical interests, you could set this in a number of eras.
*The Continental War: Taking a page from Jonathan Strange and Mr. Norrell, perhaps early or proto-Hogwarts has to consider its position when war breaks out between England and France. Napoleon has installed a sympathetic headmaster at Beauxbatons and eventually they become involved, even bringing Durmstrang into the fold. The Ministry of Magic, still in a proto-stage becomes divided. The players must try to keep Hogwarts neutral or in another kind of campaign, they become agents on the continent, trying to rescue sympathetic wizards and bring them to England.
*Against the Ministry: We have to have some pretty dark episodes in the history of Magic. There's indications that early wizards took pretty significant liberties with things- consider the Centaurs' reactions, the House Elves evolution into servants. Azkaban wasn't built for Voldemort; it existed long before that. They had rules and restrictions in place which seems to suggest other dark periods. Even the youthful days of Dumbledore's he was thinking about power. As presented, this seems to harken back to a time when wizards strode the earth with some mastery. Perhaps wizards of an earlier period did exert dominace over a Muggle populace. So it could be very easy to run a campaign, borrowing from Ars and in the HP universe with the PCs working against a dark “Establishment” in the Ministry of Magic.
Alternately: Hacking Mage
On the other hand, one could also make an argument for Mage: The Sorcerers Crusade as a source. However in that case you'd have to come up with a fix which deals with the questions of the cosmology and the whole Reality War. My solution would be two-fold but built around the idea that both sides essentially lost in the war. The order of Reason eventually achieved their goal of creating a single static reality, but in the process rendered themselves mundane. They became the non-magic scientific and intellectual Muggles of the world. The Covenants, on the other hand, also lost the ability to see multiple paradigms. They fell into rote and became conventional spell-casters operating under a very limited set of strictures and rules. No longer do they reshape reality, but instead work through spells and rules. I'm not sure what's gained by this approach, but if someone had a particular attachment.
OK, I take that back. Here's another wild “What If” HP game which comes out of that. Let's say that the Reality War ends in a stalemate- with both sides having written themselves into the corner. Then a plot for a modern Harry Potter game could be students discovering some discrepancies in the history of the Ministry and the Wizards' order. Eventually they discover that wizards once had access to wild and crazy powers-- and could reshape reality. The magic they practice today remains a pale shadow of that. Essentially, the order of Hermes rose to become dominant. The players start to seek out that magic, possibly opposed by the Ministry of Magic who believe it could undermine the powerful structures they work from; Deatheaters might be seeking it out to revive their lost master; perhaps their remain a few avengers of the Order of Reason/Technocracy who would seek to stop them-- and what about Paradox itself?
OK, I'll admit to follow that you kind of have to be a Mage: the Ascension or Sorcerers Crusade goober, so let's return to the main idea.
From the New World
Let's also consider that we don't see anything of the New World in the Harry Potter books-- for good reason. Much like bringing in considerations of the whole wizard natural superiority vs. Muggles thing, America brings up some interesting questions about history and time. So let's consider a couple of hacks or versions of this.
One approach would be to present a newly founded school campaign in the New World game, with the late 18th or early 19th century as a target. We probably have a facility located somewhere in New England. A grand mansion in the woods or perhaps a castle carried over brick-by-brick from Scotland. Some things which have a distinct character, like Elizabeth Bear's New Amsterdam and Caleb Carr's The Alienist, are probably set too far forward. We could go even Midwestern, with something like the community of wizards suggested in John Bellairs' The House with a Clock in Its Walls. I like the idea of a Minnesotan school for wizards at the turn of the century or earlier.
If we set it pretty early we can have the adventures of a group of students heading to the New World to establish a new academy. Perhaps they're fleeing some rules or restrictions from the standard magical orders, which we'll simply call the Ministry to cover the whole of it. Perhaps the Ministry's in one of those particularly dark times, with a strong movement towards temporal power and control. This kind of game we might borrow themes or material from games like Colonial Gothic or Northern Crown. There's a lot of room there for players to really set the stage for an American school (especially with the question of why, by the time of HP, we don't hear about them?)
If we want to go further west and up in history, then consider a school in California-- with some of the influences of the old west. There's that late gold rush period and the building of various scholarly institutions there. While a little too fanciful, you could borrow from Castle Falkenstein's Six-Guns & Sorcery sourcebook or lift some ideas from Deadlands. Maybe American mages, like Hucksters use a different device from wands with the cards.
That of course brings us to a more modern era in the New World. What does that look like in the HP universe. You could have a broken community, outside of the influence of the Ministry, a badlands. For a thematically suggestive book on this view, consider Peter Straub's Shadowlands. Perhaps instead there's a kind of corporatist approach to magic there. Imagine research triangle for wizards. In this case I'd borrow liberally from Monday Begins on Saturday and Stanislaw Lem.
But I've spun out a little far from my starting point. I think if I wanted to mash up Ars Magica with a Harry Potter game, I'd go with a pretty simple system. I'd borrow the ideas of Ars (research, certamen, the Houses, the magic structures) and put it into something easier like Action Cards, Risus, Savage Worlds, BRP or the like. The important thing would be to get the flavor across, more than the system details.