To address an issue brought up earlier in the thread:
Although I can't really be sure, I still think a large portion of players only achieve the minimum requirement before letting everyone else complete the rest of the challenge for them.
My suggestion is to make tiered requirements; each tier corresponding to a particular reward chest.
For example, it could take 2 personally completed quests for you to open the yellow chest once the community reaches that point, 5 quests to open the blue, and 10 to open the red.
I understand that many people play infrequently, or don't have time to spend hours ingame, but these requirements don't have to go overboard.
Besides, if you didn't spend that long participating in the community challenge, you don't deserve the higher rewards.
I just don't like the thought of people only completing 2 quests earning a legendary. A reward like that should require more effort than that. Having tiered rewards would encourage further participation, without completely disregarding those who aren't online that often.