Every time I play other game I see this implemented which makes me wonder how difficult this could be to be brought to SE. This, added to a daily quest system should be a great improvement to the game and I doubt is too hard to code.
I bet is not even necessary to add many features, just that some tag names will be exclusive for guild members, also automatic sleeves and play mats for the guild members with the same cost but without having to ask to support. A guild weekly raid could work too against a random boss but this last option is not important either.