In a relatively recent post I made mention to Harry Potter. Since I did that I have been in correspondence via email about those books with someone. I don't hate them but also do not think the deserve all the hype that they received and I did not like the last few. I have read them both because of the hype and because a couple of family members were reading them.
Harry Potter is not a series in which I read much fan fiction of but I have read probably more than I would like to admit to. I do not seek out Harry Potter stories as I do with other fandoms such as Ranma, Teen Titans, Buffy the Vampire Slayer, etc. The fics that I have read were read because they were A. suggested to me by someone that at least generally has good taste, B. are in a crossover with a series I enjoy (such as Ranma or Buffy the Vampire Slayer), or C. they were written/favored by an author whose work in other fandoms I enjoyed. Just because I did not like the books as much as some people think I should does not mean that there aren't some very good fanfics in that fandom (given the sheer number it is practically guaranteed that there are some good ones amongst the countless terrible or at least uninteresting ones).
In these emails a possible Ranma/Harry Potter Crossover was discussed. Below are my opinions on 10 things discussed (there are more which I may or may not post on later - probably will at least a couple since I have already started them in draft but that isn't a surety).
1. What year should the crossover occur were it to be a direct crossover (i.e. both worlds are the same)?: I suggest after 1994 because Ranma was pretending to be Ryoga's maid and annoying Ryoga in 1994 according to the only calendar seen in the manga. I'd probably choose for Ranma to show up at Hogwarts in 1995 after the summer when the DA is formed (this is in part because I disliked the later books more than the earlier ones).
2. Dumbledore is portrayed to be manipulative. In general Ranma is not portrayed to be manipulative in fan fics even though he is extremely so in the manga though his machinations are usually short term. This can be displayed in a variety ways such as Ranma joining Hermione's organization S.P.E.W as a means of manipulating her into helping him (also could be used to get her to think the way he wants her to think via pointing things out like if you want them to give money sell Okonomiyaki or something). Her desire and inability to really create or do anything for her organization could easily be used as a means to get her loyalty. This works particularly because Ranma dislikes people (which would probably count House Elves) suffering/being hurt, will help people if asked or if he thinks he should, and he would view it as a way to get her to do what he wants (he is quick to notice and exploit any advantage he can in the manga).
3. Ranma being near dementors would likely be detrimental to every single person anywhere near him. Dementors suck out joy and make you live your worst nightmares. Shishi Höködan which is known to Ranma but not commonly used is powered by depression and his worst nightmares are likely to cause him to enter the Neko-ken a state in which he is capable of cutting through pretty much anything.
4. Ranma in a straight fight versus pretty much anyone in the Harry Potter universe would result in Ranma winning. The Harry Potter wizards really don't stand a chance versus him under normal circumstances much less so when Ranma learns some of what they can do even if he can't do what they can. This is one of the main reasons as to why I thought the crossover wouldn't work well. There are ways to work around this and ways to make it still work but Ranma versus Voldemort and all his death eaters at once I'd still give Ranma better odds of winning. Hell Akane versus Voldemort I'd give better odds for her than him especially were she to have seen a wizarding duel prior to encountering him. That is just my opinion though but it is based on what is shown namely tossing one spell at a time, the wands are easily seen used to launch spells which can be knocked out of their hands or broken and the way that characters had dodged spells in the books.
5. Harry Potter is sometimes in fics displayed as a prankster usually because he hangs with the Weasley Twins on occasion or because his dad and friends were. In the actual books he is not a prankster and it would be out of character for him to do so especially if the other person hadn't done something to him. Ranma is more of a prankster than he is though this is generally not displayed in fan fics. Ranma's most common pranks are the use of ventriloquism (for example on Ryoga when she said kiss me in presumably Akane's voice), disguises (like how he pretended to be Ryoga's maid and messed with Ryoga for awhile), and sneaking up on people to see their reaction (most commonly Akane but has done so to others like Cologne, Gosungki, and Happosai as well).
6. Like Ranma the Harry Potter fandom is full of fanon such as Hermione's parents names they like the Tendo's deceased matriarch were never given first names or most everything on some characters like Daphne Greengrass who was only briefly mentioned in one of the books. The use of fanon to some degree is pretty much unavoidable but it is my opinion that some attempt to avoid using common fanon should be made.
7. Starting the crossover later in the series means that Ranma would be behind all the characters, for the most part, in classes on magic. Deaging Ranma and having him start at the beginning is the most common solution for this problem. That is by no means the only solution, particularly when you take into account the existence of time tuners. That said Ranma has no reason or desire to learn some subjects and has no need to learn everything every class has. If Ranma studies it like he does martial arts (hours on end straight through the night for days at a time learning it well enough to be able to match people who had been practicing the style for years) several years worth of studying could be compressed into a matter of months. Astronomy could probably be skipped entirely; considering how a Japanese highschool teaches math arithmancy would not be all that difficult; muggles studies would be a waste of time (I'm also of the opinion judging by some of the Wizards complete lack of knowledge that class would be highly inaccurate also what Ranma considers normal probably differs significantly from both muggles and wizards); history of magic could be substituted by a book with the highlights; divination is mostly an art that Ranma already has somewhat and makes use of (danger sense which includes warning him about indirect dangers like the magic Sakura Mochi Akane made); Potions, Herbology, and care for magical creatures do not need to be taught completely since after the basics are known carrying reference books and using them as needed would suffice for most situations (once you know the basics on cooking a cook book is generally all you need to later make recipes). A lot of the stuff taught in schools is useless trivia that will never be used, school is also extremely repetitive going over things far more often than the people I know ever needed, etc. cut the filler, trivia, and busy work teaching mainly that which is needed to be known and the amount of time needed to do so goes down significantly. Encyclopedic knowledge of spells and stuff taught is not needed (that is Hermione's thing).
8. “Brilliant,” said Hermione. “This isn’t magic — it’s logic — a puzzle. A lot of the greatest wizards haven’t got an ounce of logic, they’d be stuck in here forever.” Remembering that quote is a good idea since a lot of things done by the wizarding community do not make much sense such as allowing the Goblins control of all the currency when we are told that A. The Goblins are very greedy, B. The Goblins had periodically risen up and gone to war with the Wizards, C. The Goblins are resentful of the wizards for the actions of the wizards like not being allowed to carry wands, D. the economy requires money and the Goblins can easily mess things up since they control the economy to a significant extent. That is also in part why Voldemort was capable of getting support from non-human magical creatures, he treated them better than the ministry did. Ranma has a tendency to think out side the box (he will exploit any weaknesses and make use of loopholes), is rash/impulsive, but still has more sense and treats supernatural creatures better than the average wizard. Remembering that quote I find to be extremely useful when I read the books because it is how I explained some stuff that didn't make sense.
9. I rarely care for and usually ignore things in interviews (my view is that only what is actually in the source material counts anything said elsewhere maybe of interest and while having more authority than fanon is still not canon). I would use some things said by the authors. I would like to point out that people who can do magic do not always get a chance to since some likely can not afford to be taught, not every country in the world probably does things the same, some can be disallowed from using it, and this quote from the author - There is a character who, in desperate circumstances, manages to do magic quite late in life. This is very rare. [quote from Barnes & Noble chat #1, 1999]. I mainly bring this up to point out why Ranma may not have been contacted at some point before about the ability to use magic - late bloomer, different means of contact, something Happosai did, etc. can be used to explain it were that route be chosen.
10. There are a number of inconsistencies within the books and her comments for example from her comments she has stated that there is something like 3000 wizards and witches in Great Britain. Other statements include saying that there was around a thousand students at Hogwarts which was in a later statement said to be 600 students. Wizards are said to live longer than muggles so the numbers really do not makes sense. 20% of all witches and wizards in the country are between the ages of 11 and 18 and the muggles out number magic users at more than 25,000 to 1 in the UK. Inconsistencies in the numbers both in the work themselves and via comments made elsewhere by her are not the only inconsistencies for example in Harry Potter And The Order Of The Phoenix Harry, Hermione, and Ron take seats in the back row of the Quidditch stadium. A moment later Hagrid approaches them coming through the nonexistent rows behind them. I would pretty much ignore any perceived inconsistency (like those seen in the Gambling King arc of the manga with Ranma betting something he didn't know about for another decade) and I would take liberties with the story to better suit my story (while I advocate avoiding common fanon I also do not believe you have to keep everything as it was in canon).