Hey so. I essentially was thinking about this when I would want to represent multiple guilds but couldn’t. The idea is to have alliances. Essentially, a group of guilds would come together to sign an Alliance. Then there would be an alliance chat option where everyone in all the guilds could talk to each other. There could even be alliance perks like there are guild perks. The alliance perks could potentially be automatic. For instance an alliance would have milestones that would be reached by getting X alliance influence or something. This could be done so that there is no alliance leader so that having a guild leaving or joining would be an easy task. An alliance could have a cap of how many guilds can be in one. Guilds would still retain their identity while allowing for a chat system for multiple guilds to use.
That or just add custom chat channels like City of Heroes did (R.I.P. City of Heroes).