Piggybacking off this post: I believe a way to handle guild island would be to allow guilds to obtain their own, customizable islands at a cost.
However, to avoid adding more work for staff, the customization part could be entirely a guild's responsibility, coming at a sizable pokedollar/cc price (which also serves to address inflation and increase guild loyalty, as well as help the game's finances).
One way that involves little staff work would be to make guilds design and submit their own islands, under specified constraints (dimensions, amount of objects, etc). This will add an interesting activity for all guilds in the form of map editing, where guilds can participate in their own map design contest and generally have fun playing around with map editor to have their own unique islands. Having something that is unique to your guild certainly helps people identify with their guild more, and make it a more active scene than it is now. This also could introduce more players to map editor and nurture future PRO map editor staff candidates, or people to help out with new event/region designs.
These custom maps, and any changes associated with them, can be uploaded with major patches every month or two, meaning its low maintenance. All that needs to be done would be to upload/change map files that the guild submits under specified constraints. Additionally, the map size, amount of zones/areas, number of buildings, and usage of specific decoration/furnitures/ornaments can be scaled to both the amount of guild donation (either CC/Pokedollars or both) AND rank. That way, PvP guilds still get the prestige, while PvE or casual guilds can also enjoy it in their own terms. Meanwhile the CC/Pokedoller cost will also help reduce inflation overall.
If guild islands are individual as such, they can also have common areas where people from other guilds/non guild people can enter to have a look around, which will allow guilds to have another way of showing off their prestige and incentivize improvements to their respective guild islands.
Just a rough idea, but i think having a special map accessible from major ports, (maybe call it guild terminal), with stalls representing each guilds with islands that directly takes you to their guild island, would be a great way to initiate this. And the respective guild islands will have the main common area where everyone has access to, and side zones that are private to the guild (where the other guild businesses, move tutors, etc are taken care of).