I know it is a small thing, but I would guess it is a simple thing to add. 

I am not sure if anyone suggested it, but it would be very convenient to have an alert when it is your turn.

I have played multiplayer in windowed mode and was browsing at the same time. What happened was that we made each other wait, a lot. I simply could not hear the very light and quiet sound when it is your turn.

It can be added to options to be turned off/on, some sort of loud and clearly visible alert.