Here is a suggestion that I think might be interesting to implement.

When a city's production queue goes empty, please list any bonuses that the city gets for having a empty production queue in the Queue box.

Often I feel the need to build something just to not lose the production, it would be valuable to see if doing nothing is better than building a building just because I can.

Other suggestions for Empty production queue's rather than building units or buildings:

Allow a city to focus production on maximizing one of these areas:

Gildar - a city that chooses this gets a 10% bonus to Gildar production (10 metal is consumed per turn)

Unrest - a city that chooses to focus on reducing unrest gets a -10% to unrest in the city (there is a 10% reduction in grain per turn). However, this assumes that you actually make Unrest a real game mechanic, otherwise this will never get used.

Research - a city that chooses to focus on Research gets a 10% bonus to Research (10 crystal is consumed per turn). 10 might be too high, perhaps 5 cyrstal.

So if the queue is empty, it gets it's normal bonus. If you attempt to focus on one of the above three areas, you get your empty queue bonus IN ADDITION to the focus benefit.