First Off, I just wanna say that I love the guild missions addition so this topic isn’t anything negative in that respect.
My suggestion is to implement something similar to alliances from GW1. The setup could be similar in many respects as far as the UI is concerned. Shared alliance chat is an obvious thing to have. The main thing would be the possibility for alliance members to help allies with guild missions and still earn the good PERSONAL rewards they would get when earning for their own guild. Maybe even have an alliance influence stat so there is extra incentive if alliance members have already earned their commendations or some other type of incentives like laurels as rewards.
This could also have a great effect on WvW play with smaller guilds being able to either team up to coordinate or maybe be in an alliance with a large guild that is better at organizing. The shared alliance chat would be great in this respect.
I’m sure this idea has been brought up before, but I just wanted to bring it up again because I see a HUGE need for them now with guild missions and how it relates to smaller, close knit guilds.