I've oft wished for a system where FKs could be used to automatically mark rows as invalid instead of deleting them, and you could inspect the nature of the bro

I've oft wished for a system where FKs could be used to automatically mark rows as invalid instead of deleting them, and you could inspect the nature of the broken reference as a property of the row/object. Then broken references can be cleaned up when appropriate, possibly never depending on the application. This gives the neat property of being able to do data entry in parallel, too - different people may insert records into two tables linked by a FK relationship, it's nice for this to happen in either order. It also allows the user to query for exactly what rows will be affected by a cascaded delete, or to perform the operation in steps.