Not sure if this has been suggested before, but I've seen it DONE before, and it works out okay.
There's currently only 3 accessible guild dungeons, and they're only available to 3 guilds. Exclusive drops on this server are customized to be given by the monsters that are within them.
Suggestion: Make guild dungeons accessible to people-who-don't-join-WoE-Guilds similar to the way that Fairy Land works--if you want access to them and aren't in a guild that has a castle, you'll have to pay a certain amount of zeny for each trip--the price of entry would need to be paid each time you go in. But those who ARE in the castle-holding guild still enter for free.
This CERTAINLY has
nothing to do with not wanting to pay 10 million+ zeny for a linen glove that had a 3% droprate??