With the changing of the messaging system I don't have an exact way to do this now, but we'll need something to be able to send messages quickly to one another.
I was thinking something like, when you click or hover on another citizen's avatar somewhere in the site, a small icon comes up, 1 click and you can already write your message in a floating modal, another click and it's sent.
On the receiving end -if we are to merge with Invision anyway-, the message could be sent straight to the Invision api, and a feedback loop is needed too so you get notified in-game, thus if you have a new message on Invision, we could have a mail icon popping up in the top nav bar, with a small number indicating the unread messages.
ps. actually we could merge the trade feature with this quick messaging later, so you could not only send a simple message to one another, but assets/currency as well. I imagine a lotro style system, you could simply drag and drop assets into the message itself. (then the trade page could be dropped too).