There is an SIMPLE solution to this problem.
First we increase the number of guild coins you get for completing your task. Secondly at the end of the day, the guild quest is finished, and everyone who participated in this guild quest get an bonus in guild coins. If you didn't participated but you did participate in four times out of seven times you get half that bonus. Then a new guild quest is started.
This will make sure you get one guild quest every day. Also those who participate get more guild coins then those who have to miss this one, but still if you are doing it often enough you get some guild coins. Those who never do an guild quest or only very rarely will only get the coins when they contribute.