I've read a couple of suggestions on house keys, and have to say I think it would be a great addition to be able to give other players access to your house. (imagine for holidays, but also in an economic way: Become a gardener for example).
I've read Tiny Speck is going to develop a 'house key' system. But don't want it to interfere or cause confusion with the existing keys. I'm totally with you! Therefore I came
up with an alternative system:
Wouldn't is be awesome if the 'grant access' functionality would work through Garden Gnomes? They have zero functionality at the moment, and I think it would be an easy way to grant friends access to my place. i just tell my Gnome which player is allowed in, and my Gnome will grant him access. Otherwhise the door stays locked.
What I like about this system:
1. Gnomes get functionality.
2. Revoking access is easy and fully controlled: Just tell your Gnome! No hassle asking keys back etc.
3. No interference with the other key system.
4. No need to implement weird restrictions or side-rules, like: House keys can't be auctioned, or passed through. This would only bring confusion.
I know you're already working on a house key system, but wanted to drop this idea anyway :)