The one I'd love to tell the planner is that a table holds transactions in time, and that it should not expect that today's data is empty because it was empty 10 hours ago. It's an extremely common pattern, it makes any statistics gathering based on percentage of data changed dubious pretty quickly, and harms a whole lot of real queries, because in data like this, people care the most about the recent data.

There are ways to organize data to minimize the issue, but it'd be so much nicer if we could just teach the optimizer that this is the way the data is shaped.