As long as we have plenty of factions now, and this number grows steadily, I thought about an event for guilds concerning factions. It’s “Chamber of Secrets” (cut those HP associations off).
The most simple way: an extra room is added to a specific faction, and all guilds fight in this faction to rob that room and successfully finish the delve. For example, “Hall of Guardians” opens an extra room behind one at the south-west (level II), and this room has some treasures like traitstones, gold, chaos shards, glory etc., but they are available to you only when you finish the last room (level VI). Progress of the event is measured with points, like in usual faction assaults.
A bit more complex: the faction with an extra room is unknown at first, and your guild needs to find it.