This is a feature you will find in many games as it works well with RTS games. Many games takes it a step further, and fully animate buildings when they are active–and although I love that idea more, this idea is extremely cheap and easy to implement. Taking that into consideration, here are the benefits that justify adding it into the game;
Remember when we had no global queue, and had to depend on clicking on individual buildings to see if they were producing? That was rough, and it is great to have a global queue now. However, we can take this one step further by adding lights from within the building, shining through their doors and windows to showcase that they are active. This would directly tell the player if a production, tech, religious building is active or not, by simply glancing at it.
This is not meant to be a replacement for the global queue, but simply add even more visual context that helps players feel attuned to the gameplay by using their basic senses. This would also be a mechanic to be wary of in combat, as a player could change their strategies depending on if the enemy production buildings are active or not. Of course it could mean that for instance, those players could camp an active production building, but also that the other player could fake production in an attempt to stall out the enemy, so as a gameplay mechanic, it can be utilized in many ways.
It also grant more life to the currently stagnant cities we build. Consider that the things that move the most are either villagers, mills or a lumber camps. Seeing faint lights coming from inside buildings going on and off, it would contribute to a city that feels more alive.
Here are two mock ups I quickly put together.
Barracks
Stables
I didn’t put too much effort into these, so don’t take it at face value. It would be up to Relic to decide how to fully implement it, how strong the lights should be, what colour and if they flicker and so on.
Anyways, what do you think of this idea? I feel that it is a reasonable enough considering how easy it would be to implement as well as how much it would add to the game feel both in gameplay and visuals.