Implement a single, identical "Household ID" which is shared by all members of a household.
This doesn't currently exist. There is a "relationship import ID", but it is reciprocal between spouses/other relationships and thus cannot be used in the same way an identical Household ID could be.
This would be so helpful for things like mailings, reporting on household/spousal giving, etc.
I'm surprised this isn't more highly voted. Every other CRM I've used had this and it makes a lot of things easier.