Open world? Original Characters and setting? Large Group?

I've been trying to look for a first ever advanced RP to join, and I think this could be the one. My one weakness is actually planning development and arcs. I've had plenty of development and arcs with my characters in Casual RP but I never, ever actually planned ANY of that, and it was a gradual, organic thing rather than every single post doing something to further both their arc and their development.