Not sure if this has been suggested before or whether it even qualifies as OoL but here goes: guest passes for home instance!
Basic functionality would be like a guild hall but far more restrictive. A database (Gem store item possibly) that will record the accounts that can visit any home instance they have been granted access to without home owner actually being there. Additional licenses can be sold separately for increasing the total number of visitors permitted. Members could be added or removed much like guild log. Bonus points if these authorized visitors can bring in others via party when they open instance.