Estate keys or guest lists would be a great way to share access to estate with friends while sidestepping the issue of Eternal Bond and divorce. With Free Company housing, most players already understand that any items they place in the shared structure could be moved or destroyed by other members, and that private rooms may be demolished without refund to them. Certainly, some changes to this may be appropriate but this is simply to state that players currently accept that placed items are a "donation" and accept the potential losses.
What's great is that this lets more people enjoy visiting their friends houses instead of only being able to share with one person and only if you bond with them. On the other hand you don't want to automatically give access to everyone on your friends list, you want to be comfortable accepting new friends requests without worrying they will munch on your plants or come in uninvited as you are making lewd screenshots in your estate.
For me the most important reason for having this are my friends who have plants in their personal house and come into a situation where they can't log in for 24 hours. With the gardening feature it is especially important to be able to let someone harvest your plants for you before they wilt away. Although... I can see some players using this feature to "rent" out their unused private gardens >.>
Another bonus to these approaches is that similar features are already in the game, which makes it intuitive for players to understand and easierto develop. The keys in Sastasha and Haukke grant access to specific doors, so players know how they work. Eternal Bond invitation items store information about what time they are for and are removed from player inventories automatically when they are no longer valid; the estate key item can be modeled after these. Linkshells and Free Companies have rosters with configurable ranks and permissions; a simple guest list can be cloned off of Linkshells and stored the same way.
House keys
Players need to purchase a special door or lock, then purchase keys like the blank Eternal Bond invitations, which can be traded or moogle mailed to their friends. Possession of the key grants the ability to enter a locked estate, and additional housing permissions not normally available to guests such as harvesting crops, which also apply if the estate is unlocked. At the beginning, this can be made easier by granting a fixed set of permissions to every key holder: place/move furnishings, harvest crops, but not demolish the estate or change its name. As the feature evolves, the ability to set permissions for individual keys or key types can added. To revoke access, destroy the lock and all keys are invalidated.
Advantages: Easy to understand. Fun giving keys to your friend as a gift. If you are given a key, you know you've been given access. Friends might be able trade it back to the owner for reuse.
Disadvantage: Player inventories will be filled up faster - some will own 20 house keys, may need to figure out a way to make these untradeable upon receipt to deter costly house vandalism, ability to manage permissions may not be as configurable as when using a guest list.
The best idea if you need to add another gil/seal sink! Make all keys destroy on any lock change, so estate owners with 40 guests who really want to kick one person out must buy 39 new keys. If keys are used do NOT allow keys to be individually revoked or removed from specific players by GM intervention, estate owners are typically wealthy players with millions of gil on hand, making them pay 5,000 gil per key (100,000g for 20 people) or 5,000 GC seals for a new door is nothing.
Guest list
A list of players who are granted the ability to enter estates when they are locked and also receive additional permissions. The players are selected from the Friends List like moogle mail recipients or invited like with Linkshell invites. As use of the feature evolves, ranks can be added like the ability to specify Linkshell leaders.
Advantages: Doesn't require player inventories to be filled up. No trading of keys. Easy to see who has access.
Disadvantages: Not as fun as trading items, might take more development time, whatever other reasons a guest list wasn't used for Eternal Bond invitations.
Can still be a moderate gil sink if you make estate owners buy a new door or lock to access this feature, and possibly limit list size by capacity tiers with a requirement to demolish the old door and buy a new housing item as 10, 20 or 50 people are reached.
Posting here instead of housing forum because this is where theBalmunghousing complaints always seem to be