Your ideas are interesting, but I feel like if there is no base conflict that either you or someone else makes would make the RP slightly bland and it won't continue for as long. For as long as i have been here, adventure RPs or RPs that have quite a bit of action continue a lot longer than slice of life RPs that have no conflict or have no end goal.
While I like the idea of a plain school RP, I feel like it won't have enough substance unless you make something bad happen at the school or something about the school changes drastically and each student has to cope with either changes in staff or changes in rules (of course, only when it kicks off and has almost a page of replies, or maybe just go through a full in-RP day would be great to implement this).
The magic school idea is interesting (and since I'm slightly more biased towards fiction, I like it a lot more than the first idea), but like I said, if you don't have some conflict in the RP, it will quickly grow stale. Maybe there is a bit of racism going on in the RP due to a ruthless gang that can't be taken care of because there is no solid evidence to stop them, or maybe there are just awful cliques due to these people feeling like they are separated between one another, or maybe there is a slight power struggle between the top 5 students in school (these ideas could work for the first idea, too). Or something bad happens at the school and the kids are supposed to use what they have learned to stop the threat, only to realize that it was all staged by the staff and no one was really in real danger.
The power school RP reminds me a lot of the likes of My Hero Academia, Soul Eater, and other such power school RPs. I actually wouldn't mind seeing another one of these appear, but you'd have to have something that separates it from the rest of the others to make it more like your RP.
That being said with all of these school RPs, you can always have your future players making conflicts for their own characters and make their own little arcs within your RP, which is lovely when it happens. I just don't see a lot of that happening anymore, as they tend to just follow the direction the RP creator takes them.
Now I havent seen this particular Ghibli movie, but if it is set in the same universe and you don't meet the original cast, then that is golden. I wouldn't want there to be canon characters being portrayed in an RP because of how others may portray the canon characters, so having another area set for the adventure would be great.
Now for my ideas. Since I have a feeling you are more about slice of life RPs and character development in your RPs, here are a few ideas that I have. You don't have to use them, as these are suggestions.
- A Vampire/Werewolf RP (and other such creatures if you'd like, just as long as they look vaguely humanoid): the idea for this RP is that the humans have no idea these species exist and these species want to keep it that way. There are tensions between the vampires and werewolves and other creatures and the humans are caught in the middle. You can make this more action or adventure oriented as you like, nothing has to end in bloodshed.
- An Adventure for Treasure: a ragtag group of people, who are unlikely to even be talking to one another, are forced into a group for whatever reason as they find a treasure or a creature or a person they need to complete their own little goals. There will be some conflict in the group as there doesnt have to be anything dangerous coming to fight them.
- Mercenary School RP: Mercenaries don't have to come from the depths of a jungle after being left there for years and get tough from that. They can actually be educated on the essential do's and don'ts of what it takes to be a mercenary. Once students come in, they better be mentally and physically prepared to take on gruelling tasks to toughen their bodies and minds, or else they'll find themselves wake up near their house with no memories of how they got there and where they were. They have options to choose between different weapons (including more unconventional weapons due to guns being overused). These mercs don't have to be average Joes with the penchant for bloodlust. They can be other creatures or people with powers, as well. Because people dont want to leave and have to find other things to start their lives over, there will be a lot of competition in the air. If you actually want to incorporate this in some way, I can help with that.
- Celebrations gone wrong: a festival or a party happens and then something bad goes horribly wrong. It's up to the characters to find out what is wrong and work together, even though they would much rather party back at home than to deal with the issue. You can make the problem be anything you wish, whether it be a zombie outbreak or just finding a bunch of hooligans that ruined it for everyone.