I can do whatever with the setting, but I'm wondering how to make the mechanics match more closely.
I was thinking that the main thing to change would be requiring all magi to be a sort of Verditius-Ex Miscellanea hybrid, requiring casting tools (in this case, required to be a wand) and getting the Weak Spontaneous Magic Flaw for "free" in exchange for getting a free Major Virtue pertaining to your special wand (which, I figure, will be similar to a Faerie "Grant" item, containing numeric bonuses to certain things as well as being able to store Minor Hermetic Virtues, making it a powerful item for its wielder indeed) and a free Minor Virtue giving you either the Items of Quality Mystery or perhaps something like Method Caster that boosts your casting total on account of your wand channeling your magic. With using magic sans a wand made extremely powerless (indeed, wandless magic is possible but highly difficult and much weaker in Harry Potter) and all magi being forced to use a wand, things become rather similar indeed.
Other changes that might need to be made... Perhaps a Breakthrough has been achieved by this point in time allowing the teaching of Arts in a classroom setting? If so, you could even realistically shorten the apprenticeship to the seven years that magical institutions teach for in the Harry Potter books; although the "apprentices" will only be taught for one season a year by any particular magus, there might be specialists of different kinds teaching them different things from season to season. Sure, this removes the benefit of taking apprentices from the potential teachers, but if all magi can be trained in one school, there won't need to be a whole lot of teachers anyway, so there will surely be enough willing to take a season out of each of their years ensuring that the next generation of magi is fully taught.
Based on the guidelines in Apprentices, and factoring in the lack of +6 SQ since the students are not being taught one-on-one, the average SQ for each class season will be 7. 74 is 28 xp plus 4modifiers per year; we'll say 4, since those modifiers only apply (and for that matter would be a really huge deal) if the apprenticeship is played through. 287 is 197... Hm. Okay, so 7 years isn't quite long enough to get a real education... Unless the requirements for teaching credentials are bumped up? If the people usually reserved for "Skilled Parens" are the teaching magi in this situation, the Source Quality gets bumped up to 13. 134 is 52, 52*7 is 364... Teach them 120 levels of spells as usual on a 1 xp to 1 spell level ratio, and you've just about got the standard apprenticeship's worth of everything. Sweet. I might want to boost that up a lot, actually, because Harry Potter wizards focus on a lot of practical skills alongside magic, but this is one of those stylistic choices I'll find myself prattling on about throughout this post.
As more of a way to stay true to the Harry Potter's style, I think it might be a good idea to have the Master of (Form) and Extractor of (Form) Vis Virtues (all 20 of them, yes! All from RoP:M if you're wondering) integrated directly into Magic Theory and cut down on or completely remove vis from the equation. Okay, maybe not completely remove... It doesn't exactly work to have magic items cap out at level 20. But at the same time, enchanting items in Harry Potter is more of a "pouring yourself into the item" sort of deal, and given how much the Harry Potter series focuses on the magic being within the character and their wand, finding little puddles of magic in slain monsters or growing out of special locations just seems... Sort of wrong. I mean, it can totally work, and since a school (or covenant, of course; this will still be somewhat like Ars Magica after school's done) like Hogwarts or Beauxbatons or Durmstrang would certainly exist within a rather strong Magic Aura (there are no mundanes around within the school itself in most cases, though they'd likely farm or engage in whatever method of acquiring currency the school employs just outside the Aura) they could probably just farm the vis needed for any item enchantments... Though that might be a rather time-consuming approach depending on how far over your Fatigue limits you need to go. I'm on the line about this one; give me some opinions.
Importantly, wizards in Harry Potter neither had Magic Resistance nor any trouble dealing with mundanes, as mundane "infiltration" was often necessary. That may not be the case in Mythic Europe, of course; indeed, given the rules already available in Ars Magica, it'd be simple enough to replace HP's Wizard Duels with Certamen and accept that they'll be outcasts in most societies for more reasons than just frequent Turbulences. The bigger concern, here, is that with the new school-based magic teaching system, all the students would probably be too busy hating each other to learn anything, especially when group work is required. I was considering two possibilities. Either Parma Magica could be moved to earlier points in the teaching schedule, so the students could protect themselves from the negative effects engendered by each other... Or, simply to follow HP's canonical wizardry more closely, give everybody the equivalent of the Gentle Gift, and quite possibly remove the Parma Magica entirely. However, while removing Parma helps keep true to the series in and of itself, it comes with unfortunate implications given how the rest of how magic scales in Ars Magica. Spells similar to HP's "killing curse" are achievable at such levels as 40, and can be cast successfully (with Fatigue) at casting total 30. With no need to penetrate a Parma, it would become WAY too easy for characters to kill each other quite early on, while such magic is illegal and, more importantly, extremely difficult in the source material. Normally I'd disregard it, but the difficulty in killing adversaries is a huge part of HP's spirit, so I'm... hesitant. I was considering applying a houserule that I saw mentioned before, with Parma providing a much smaller general magic resistance but giving a huge boost to defensive spontaneous spellcasting (which would be necessary to make defensive spont'ing even plausible here, since everybody has Weak Spontaneous Magic...) equal to 5 or even 10 times your Parma Magica score. Even MR 0 prevents atrocities like casting total 30 kills, so... Opinions on the whole Gift and Parma Magica things?
Eh... Potions is an entire class in Harry Potter... Perhaps I could also have either Mythic Alchemy or, probably more accurate to HP, Single-Use Charged Items (a Learned Magician integration possibility, HMRE) already incorporated into Hermetic theory?
There are actually a lot of interesting parallels between the Ministry of Magic and the governing structure of the Order of Hermes. I probably won't have to change much at all in this regard, really. The Ministry is more aristocratic than the Tribunal system, and the Ministry's laws are a fair bit stricter, but overall they are very similar systems and the Code can pretty much stay as it is.
Another big concern, for me at least, is Warping, specifically Twilight. It's probably going to just be one of those things that's different between the novels and the games; even in HP Ars Magica, Longevity Rituals are still extending lifespans and magi are still going into Twilight and generally ceasing to exist for brief periods and hopefully coming back later with more or less knowledge. The whole Realm Warping, not to mention the Realm divide, interacts somewhat weirdly with HP, even, but a lot of it could be explained away with a simple change in terminology, so I don't really mind it all that much I guess.
I can't really think of anything else to change to turn Ars Magica into a believably Harry Potter world, myself, though there's this itch in the back of my mind that I thought of something really important before and forgot it while writing other stuff.
So, what are your opinions on the changes I've considered so far? Any suggestions for more things to change? Explanations of how stupid an idea this is and how I'm ruining the best parts of Ars Magica? Alternatively, exclamations of how fun this seems and encouragement for me to try to solidify this? Angry criticism of my poor third-language English writing skillzorz? Anything else you feel like saying after reading the ideas I'm tossing around about how to Harry Potter-iffy a variant Ars Magica setting, mostly just by changing how the Order of Hermes works (and not even in necessarily drastic ways, partly because I don't want to write a bunch of new rules for this)? If you have answers aside from "no" and "don't care" to the above questions, please voice them! Thanks in advance for any responses, whether they be encouraging, discouraging, criticizing, or even just blabbing about your feelings on anything I've written. Just as long as you don't intentionally troll, because, y'know, trolling.
EDIT: Upon second thoughts, there is Form resistance. And a standard curriculum would by necessity result in a vast majority of generalist magi. Maybe Parma isn't necessary for defense of the self...? Hm...