It could be a starting opt-in mission for new players, sent to battle in charge of a group of players against hordes, and a good way to discover the gameplay when testing the game (instead of the fetching quests or full pvp).
Eventually it would create the need to coordinate people using the [normal] and [group] chats to plan tactics and secure maps by joining forces with unknown players with a common goal, meeting different kind of leaderships that could introduce the new player to guilds and the mmo part.
The monster raids could be waves of monsters, varied sizes and quantities (game lag has decreased with the multithreading beta, maybe channels can resist mob zerg rushes without having to lockstep all ticks)
You can “fake” monster spawn portals (maintaining the concept of raid) with planes with animated textures, the monsters spawn behind and rush aggroed, or with certain structures.
Guilds could have a tower instance placed to secure the front line (or teleport).
The wave raids could take over maps if the kill quota is not met (integer count), or by losing structures.
Front line could be pushed by moving the towers against map entrances, guarding the node map, repaired by guild members (silver cost or item cost).
Lost maps could mean losing teleport access north that node.
Reconquering towns and winning maps could be pitched as history of the game.
If considered, it should be ongoing constantly and not event based, because if it is a constant feature it will put pressure on losing map access (blocked teleport areas).
Glory/exposure would be offered to guilds, in some way.
Monster wipers, testing builds and gear, could be seen as celebrities by new players
PD: Could be nice if the hordes hit hard, to the point to wipe all afkers that surely will show up.