I think this is a good idea, and that the game would be “best” if guilds had something like a forum within the game to communicate. Chat is somewhat limited and doesn’t have much structure.
On the other hand, that’s a lot of UI work and storage. I think they’d have to spend at least 2 months worth of work to implement the first version. Longer if we want them to take feedback and add features beyond those which were initially added.
We already have Discord, Skype, Facebook, and a handful of other tools for communication between groups of people. These things don’t involve extra effort for the developers, and you can do a lot with all of them. Personally, I think in-game chat is a waste of dev resources considering how well these other solutions work. Every time something goes wrong with chat (and from what I gather it’s third-party anyway), someone has to stop working on other GoW features to investigate the problem.