I love this idea. The worlds of studio Ghibli would be amazing settings for rps. The problem, of course, would be making the plot suitable for a large group of people since the movies all follow one main character. Everyone seems pretty set on The Cat Returns already (which is one of my favorites), but I might as well write down my ideas for the other movies as well. Prepare for a lot of brainstorming.
Spirited Away: For this one players could be spirits who have forgotten their names and, as a result, are forced to work in the bathhouse under Yubaba. This idea would have to rely a lot on character interactions I feel, since working at a bathhouse isn't the most exciting thing for your character to be doing. It would also require a lot of character development because of this. We would need some canon characters though, such as Yubaba and Kamaji, so that is something to keep in mind.
Howl's Moving Castle: This can take place either after or during the war. I'm partial to after just because most wizards/witches were fighting in the war as those little monster things, so it would fit the movie plot line better if it was after, but I could go either way (P.S. I have only seen the movie, so people who have read the book may have more information on this than me). Players could be wizards/witches or just normal people. Magic might get a little complicated if you wanted to limit it so characters aren't overpowered though. This wouldn't matter much without combat, but it is something to think about. As far as the story goes, it could become very dark if we wanted. Assuming it is taking place after the war, this could go in all sorts of directions. My idea is this: the prince has returned and the war has ended, but the wounds that the war has left behind have only begun to heal. Many people remain homeless, all their possessions swallowed by the fires, others injured, unable to work, and the lucky ones who escaped the bombs' reach have not been lucky enough to escape the poverty that the war has brought. In order to aid these suffering citizens the king has established a special group of witches and wizards that will act as a war relief squad. And so on and so forth.
The Cat Returns: In this one players would be able to be a cat, a human, or an imagined being (for lack of a better term) like Baron. As far as plot goes this may be tricky since the cat kingdom and human world are separate. I like your idea of cat kidnappers. What if this takes place in the future after Prince Lune has completed his reign? And the new ruler who has come to power hates humans, (perhaps he was abused or something) so he has decided to purge the world of the human race by turning them all into cats. The story can start with the latest batch of kidnappees waking up in the cat kingdom. The Cat Bureau (imagined beings) can already be there hoping to stop the cat king and, upon seeing the people who were kidnapped, explains what is happening to them. Then they team up, and the journey begins. Not the most believable plot, but hey it's a movie about talking cats.
Arrietty: I think this one would be especially fun to do, and it would be easy to fit in a lot of people. Players are either a tiny person living in the walls of a house or a normal-sized person living in the same house. Fairly self explanatory I think.
Princess Mononoke: I really like this movie, and I think it would make a great rp, but I've only seen it once years ago so someone else would have to come up with the plot. I just thought I'd mention it.
So those are my ideas. Take 'em or leave 'em. I just hope this helped.