Perhaps this has been suggested already, and I didn't look hard enough, but here's my suggestion anyway.
I think that the ability to ping players in chat would be useful, as often I have the game open in another tab or I am simply not looking at the chat when someone asks for me. This could be done by just typing @[username], like is seen in many other chats in other games. Also, a notification sound could play when a trade is requested or an alliance invite has been received.
The immediate issue that springs to mind is annoyance. Obviously you wouldn't want someone repeatedly pinging you just to annoy you. A potential counter to this could be a setting to turn on limits, e.g. 5 pings a minute, as well as a setting to turn it off entirely. Of course, blocked users wouldn't be able to ping you.
What do you guys think?