Guild message board
Certified Gameaholic
An in-game guild forum doesn’t sound too bad of an idea.
In-game forums are going to inevitably be worse than out-of-game forums, and anet are not specialised developers of forum software.
You would be much, much better using an existing solution for this, out of game, and letting folks interact with it how they prefer.
Both discord and Facebook groups are excellent choices, though they function differently. Both are commonly used by people who play MMOs, etc, for other reasons.
Dedicated web forums are terrible choices. They have extremely poor utility, and add another place people need to go hunt for things – most members will never bother, compared to seeing, eg, a post show up in the facebook feed they were reading anyway.