GDs were awesome because of the battles. People are bored because they 150d too fast. Making leveling even easier wouldn't make much sense.
I think actually are bored because leveling is boring. Monsters dropping woe supplies sounds nice for some woe guilds, but it doesn't make sense for everyone else. Why would they want to go to guild dungeons? to farm something for a minigame they don't play in a place where it is harder to farm because they have no chance to farm items solo? I can totally see guild dungeons flooded with people...
Exp, in the other hand, is desired by everyone, and if not, killing your enemies so they don't get exp is desired too. Make it so you can not flywing or gwing inside, neither use insurance or tokens, and give awesome exp. Maybe, make guild dungeons work like kinda battlegrounds rooms, where people enter in rounds, and have certain time in which they can level in awesome mobs, lose exp if they die, and get battlegrounds or KVM points if their team won. People who own the castle can enter freely, while people who don't belong to a guild with a castle have to pay to an npc which maybe the castle owner can take the zeny from later.
It sounds fun to me, and if you want, you could even add your desired supplies as an exchange for the points rewarded at the end of matches.