We currently have two message-bus like concepts; actions and notifications. For widespread use to interconnect components this needs to be made a little more generic, which would greatly help to reduce tight coupling between components through API.
The QML API for the bus could look very much like the action system:

Message {

messageType: Message.Mail.ReplytypeId
someRandomProperty: ""

}
Bus.post("mail", message)
Listener {

name: "mail"
onMessage {
  //Do stuff with message
}

}