Well, looking back from watching the movie yesterday, The Cat Returns wouldn't be easy to do. It's kinda complex to make thst into an rp. I say either Arietty or Mononoke will work well.
As Shizuka said, Arrietty is simple, and would allow us to let in a lot of people. It would be fun.
Mononoke on the other hand would be a more complicated and combat filled rp. I'd love to try it, but I'd need to warch the movie again to refresh my memory of the plot.
Im rooting for arrietty right now. :3