Imo, I'd say make ALL raids triggerable.
You need to work together to create a trigger. Should not be something that a single person could get in minutes, but should be an efford for entire guilds (Mining, killing and building needed to get the trigger).
For non-guilded people, there should at least be 1 week in total a month, where the guilds work together to create the trigger for an public raid. In exchange, the guilds that participated in the creating of the trigger, could get a part for a new trigger, wich they can use for a guildraid(randomized between parts and raid, so you won't get items for 1 certain raid only), or saved for the next public raid.
This would enchance the co-operation between guilds, and give everyone a chance.
Guilds could work together as well combining trigger materials and sharing the raid(s).
Example, an boss has an chance to drop an item, needed for an raid, and that raid, gives drops for yet another raid, or the same raid, etc.
This would give raids meaning as well. You, and others have to work for participation in a raid, making them more "special"