^That is, if True Magic is going to be included in the crossover. I know how overpowered True Magic can be in Nasuverse. I mean they have the Wizard Marshal Zeltrech capable of tapping into the energy of different dimensions. aka. Unlimited Powers.

However, in Fate/Stay Night, true magic was only seen at Heaven's Feel and only with the help of the Grail+Illya.

For this grand-scale war between the two factions, it's not as if the Wand-Mage association is completely incompetent. The British Faction "might" be, but we can assume that not all Wand Mage association are the same. As this would ultimately become an apocalyptic war between Nasuverse vs. HPverse, balancing both sides would be hard as Clock Tower mages are utterly ruthless.

However, the real core for this crossover would be the Grail War and how Harry/Shirou would cope with it and the knowledge that a certain Dark Lord is after his ass. Will he try to save everybody? Or go for the Minds of Steel end?

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Merging two MCs together only work when the merge is somewhat believable and has enough backstory to merit the merge. As somebody already said, the great thing about Shirou is his past. He has literally no past at all. So he can be anybody provided it's not something far-fetched.

 

The heights of FSN's power scale dwarf what we've seen in HP, but they are compatible. yes, "Magic Circuits" clash with HP's magic system, but it can always be hand-waved. If you want the story to be more FSN-like, suddenly everyone at Hogwarts has magic circuits, but wizards there don't know shit about them compared to Clock Tower magi. If you want it more HP-like, just say that people who learn magic at Hogwarts utilize a different kind of magic that uses neither Od nor Prana nor Magical Circuits. Certain elements in Fate work based off belief (like Reality Marbles) so it's not a stretch to say there are various brands of magic one can master (actually, a Reality Marble apparently takes no prana? I don't know how it works, but Reality Marbles do magic in a way similar to HP magic - there isn't any apparent bodily energy drain by using the technique, all that is required is a particular mindset and the right personal incantation). Also, I've never heard of "magic getting weaker the more it is known" - it's obvious if secrets are shared about magic, weaknesses about kinds of magic are common and a certain kind of magecraft is no longer effective in combat, but it's not simply "it gets weaker if people teach it freely". Magic in Fate is more deadly than magic in HP, for sure, but again, that can be hand-waved since HP wizards just use a different kind of magic with spells aimed to accomplish different things, et cetera

Fate is very compatible, because I think it offers much scarier antagonists than Voldemort can offer. Voldemort is a very not-dead wizard who likes to cast lethal spells... ocassionally. Certain heroic spirits have the power to defile the dead, change causality, and penetrate anything. Certain entities above heroic spirits require as many as several hundred or more magi to take down, if it's even possible to do so at all.

I agree there are certain elements that don't directly line up, but Fate's about as good compatability-wise as you're going to get. I don't think anyone is brazen enough to copy HP wholesale; compatability can only be so high.

 

It's not as much of a long-shot as some of you seem to think. For one, the Type-Moon universe openly exists in a multiverse. Harry Potter could simply be another dimension among thousands that is like the TMverse but not like it, existing parallel and generally undisturbed. . . until something happens to breach the walls between them.

Or, if your tastes run along more unusual lines, the HPverse is actually quite like what a great deal of mages would like the world to be like, in a number of ways. Harry Potter could be a vision of the future, where great powers were worked and the higher magicks were sacrificed to lend stability and universal knowability to the lesser ones. The High Magicks, the schools of True Magic, are all but lost in this future modern age.

At least, that is, until an ancient wish from days long since past emerges from the mists of legend to deflect the unstoppable, turn aside the inevitable, and blast a wicked madman at the height of his power to ash and dust, leaving only an orphan and a scar in it's wake.

The bargains that were made are beginning to come undone. The Old Magicks run deeper than any had guessed, and a modern myth that became a legend unto itself has forced disused gears of the cosmos to begin turning once again, just in time for the greatest and most terrible war the mundane and arcane worlds have ever seen.

The world will remember The Boy Who Lived, and it will remember He Who Must Not Be Named, even if those that live upon it forget. For it is golden light and deepest shadow, pale flesh and a burning heart, blood upon blood and innocent upon ambition, given name and purpose and will within the tales spun by mothers to their sleepy children as the embers of the fires run low.

And so the legend grows. And so the gears begin to turn once again.

Is it fate? Destiny? Mere chance that brings this confluence together?

No.

It is inevitability.