To date player satisfaction with Guild Quests is low, a way must be found to rectify this. I believe the key issue is the assignment of quests by the game.
Guild leaders are in a better position to know what their members are capable of than some automatic system, I suggest putting Guild leaders into the loop for choosing quests for the guild. My idea is a checkbox type list for the next unassigned quest. They can choose from low/medium/high reward quests.
Low level owning res, trading res, buffing people. this pays 50% reward.
Medium level adds low xp adventures to the mix. This pays 75% reward.
High level adds high xp adventures. Pays 100% rewards.
The above is just a basic idea, other players may see ways in which it can be improved.
For individual players there should be a opt out button if they choose not to do the quest. Opting out will count as completing but costs in guild coins an amount equal to the reward. I've included this since not all players are gem buyers.