Suggestion: End of Production Notification
Since there doesn't appear to be a reason to not always be building/training something in a city, there should be a end of production notification when the queue is empty. When I come back over to my city from exploring and find that it hasn't been doing anything for I don't know how long, I feel like missed out which is no fun. This would be a great way to conserve the player's mental resources for actual strategic decisions.