I've been playing this game for 7 years and housing shortages continues to plague many players. I've been fortunate to acquire a large house but many seem unable to even get a hold of a small one simply because there are so many people abusing the FC function where they build an FC, buy any house they can get and sell the FC for a lot of money.
So my suggestion is to add functionality to the sharing functions (or lack of):
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Private housing should be upgraded so you can share with 9 players instead of 3.
FC houses should be allowed to share their house with up to 2 other FC's.
This just means that while sharing, every FC can see your FC house as a location to teleport to and make use of your NPC's, retainers, foodbuffs (but stables, gardens & workshop remain off limits for those not part of the FC owning the plot due to rank restrictions of each FC otherwise colliding, unless SE adds separate restriction options for shared purposes).
When outsiders click on the vault in an FC house not owned by them, they will see the vault tied to their own FC meaning they cannot see or touch the vault items of the FC owning the plot.
If FC's want to request help, they can use alternatives such as discord.
That way the in game chat won't be spammed too much and FC behaviour will remain optimal (less chance of being overlooked when you ask your main FC for help).
Obviously what this means is that FC chat won't be mixed with other FC's to avoid it causing friction seeing as all communities are run differently and they tolerate different things in chat. So it may be wise to idle around the FC plot if you want to ask someone for help by for example asking them if they can ask their own FC if someone wants to do specific content and having them come to the garden to continue the conversation in say or yell.
The idea is to give everyone the benefits of NPC's & retainer bells + finding someone to hang around with without having to be lucky to grab a house.
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on a sidenote..
It would be helpful if SE updated the restrictions so you get to configure them for other FC's as if they were guests without colliding with their own rank restrictions, I think a nice way to do this would be to introduce new furniture on the market that gives us the same but removes all the restrictions for people you share with (meaning all FC's get to use that furniture as long as they are sharing with you. And treat these furniture as if "not yet in the garden" so you can place 3 regular gardens and 3 shared ones. It would also help if SE gave us 5 additional garden slots seeing as we lose some by default by the teleportation crystal, mailbox, stables, plantbox, training dummy which every FC obviously needs.
(obviously if a community can't get along with the people they share the house with and leadership can't find middle ground, the plot owner can decide to stop sharing it at any given time)