How about this, just a normal world, with some warps to little towns, with like a NPC shop. Like the blue building. And a Poke center with a NPC sister Joyce. Not to big towns more like 3/4 houses shop/gym/poke center. If you'd make a map with roads between them it would be fine, but if you do maybe like some events on the way to another town. There should be one world for the Gyms and map, and one for regulair survival.
And for the donators ranks like, Pokemon: Breeder, Champion, Ranger etc. Whatever the highest rank would be should get a master ball. Maybe master balls only for sale for donators, they should get the normal stuff like /back and /fly too. So like a regulair player could only get master balls from bosses and donators could buy them at a shop.
And I don't know about the gyms, but if you'd make them. It would be hard to manage since you'd need active poeple to do it. I don't know if its possable to create a NPC gymleader but that would be preee sweet. And maybe like a elite four.
Your goat guy, Orianokje.