I have been trying to play the coop missions lately, but to get players to play you have to go onto player finder and individually ask a player until you get the right amount of people needed for that mission. I know there is global chat but there is only ever a few players on there and these players have done quite a lot of the missions allready. What i am suggesting is a coop button on player finder which you click, then a drop down menu would appear with all the numbers of the coop missions. Once you have selected the mission you want to play, a message would then appear on everyones screen, for example:
Haggis is looking for 6 players to play coop 13
Once the player gets the number of people they need, they can then send a message through player finder that they have all the players they need.
I know that a lot of people will not want messages flashing up when they are playing, so a key could be bound to switch the messages on and off, say F12.