I have a great solution for leavers. It consists of two parts;
I) When someone leaves the group before the timer expires, the remaining party earns valor per bosses downed already.
II) The keystone system is gone; instead you can do any dungeon according to your highest completed at any time. If you two chest a dungeon, you can skip two levels. Let's say your highest HoA is 19, then you can attempt 20 anytime. If you two chest 20, then you can attempt 22. If you time it single chest, you can attempt 21. This way you can push without having to worry about someone leaving, dc'ing etc.