I like that fellowship potion room idea. In fact, just the idea of having more rooms (perhaps purchaseable / obtained via achievement, materials, etc?) would be really cool. Kind of like how you can do addon rooms with warframe but less infinite.
It’s also an interesting point. They basically have guest housing in-place already. If they were to essentially copy how guilds work, but separate, you could invite people to be a “guest” (akin to a guild member) or a “privileged guest” (akin to an officer). Home instances would be swapped with how fellowship halls work. Guests somehow trigger going into a “guest door” instead of their own home, bringing them to your house. For simplicity of implementation, you can be a guest of 1 house at a time. You can quit being a guest, or kicked from being a guest. By default all house chests and access to picking up alchemy items/tiles/fishing/etc is limited to owners (“guild owners”) but can be altered for different roles. Could also have an option to auto-kick people from the guest list whenever they leave.
My only concern, is how people can randomly appear in multiple guilds and swap randomly lol. Someone might be able to enter later after you thought they were kicked. Fellowships aren’t super stable.