You can already leave missions by just quitting and relaunching game, When they finally add cross district match making it will mean that you can just quit the game and not worry about spamming join to a full district. Will be honestly good when that happens because even when the new match maker was in place I was still getting really bad matching like bronze teammates against a full max rank gold stack. It was the biggest waste of time ever. I just felt bad for the bronze players on my team they had like 20 deaths or something, I can imagine they will never play the game again. In the future if I end up with shit balance like that I will just leave and rejoin district instead.