I like the improvement that was made to the Guild Quest system a while back. However I think it can be even better.
I know others have suggested more options to spend your Guild Coins on, and I agree. There need to be more options for your individual rewards. Personally I don't know if the 3x buffs are worth the effort, so after I've gotten the jolly geo I won't have any personal motivation for doing it. I still want to do it to help out guild mates though, but individual motivation is important and more options when it comes to the rewards could accomplish that.
I know that a lot of Guilds struggle with getting enough players completing the Guild Quests. A possible way to change how the system works would be to instead of having a fixed percentage of players having to complete the Guild Quest for it to finish it could be that each day the Guild Quest resets. The higher percentage of players in the Guild that has completed it on reset the better the rewards. This way Guilds still benefit from active players, but they can still compete it even if they have inactive players. That way Guilds won't feel forced to kick inactive players out as much as today, and individual players don't feel as stressed about finishing their Guild Quests.
However, what I think should be added to the system is something that makes the whole guild better. I think a lot of us that play this game and join a guild like the feeling of helping each other out. The feeling of building something together. What if the guild it self could become better? What if there was Guild XP? And for each Guild Quest completed the Guild gets Guild XP. There can be many options for what happens when the Guild levels up. It could be that players get a bonus to XP gained (perhaps 1% extra per guild level). It could be a free member slot for the guild to make it less expensive to increase the size of the Guild. It can also be other rewards. Like at a certain level the guild unlocks a free extra Trade Office slot for all its members, or whatever you at Blue Byte thinks would be nice. Look at other MMO games that have Guild leveling systems and think about how you think it could be used in The Settlers Online.