That communities have started to form (mostly in Discord) clearly shows the player base's interest in having guilds in-game. While I'm excited to see the eventual addition of guilds to AQ3D, I'd also be wary and avoid the mistakes other games have made when implementing guilds and clans.
Make guilds practical. Having a social hub is all well and good, but ultimately, if guilds have nothing to strive for or do, they're pointless.
Think big and allow for growth. Sort of in line with giving guilds something to do is giving them room to grow. Have guilds start out small and allow for them to work towards a larger size, obtain guild buffs, a guild house, do guild missions, etc. Interest in guilds will be limited to what can be done with them; many possibilities and features will mean more interest.
Make guilds be a living part of the game world. This is an idea I would rarely suggest for most games, but AQ3D could pull it off with its light-hearted dialogue and lore. If an "unofficial" community becomes so large, why not make it an official one? Let's say a guild forms, has a serious name, story, and is actively devoting itself to its members and the game. Could that guild be absorbed into the actual game lore in some way? This would be an unorthodox but potentially limitless source of incentive for people to form a successful and healthy community that all players would feel a part of.
That's really all I've got. From what I've seen in these guilds that have formed, they've got the social aspects down, from content coordination like dungeons, to fun gatherings and community events. AE just needs to provide the practical elements in-game that reward communities for doing what they've been doing.