To reinforce the idea of party required boss like monster, why dont we take a further step by making a huge structure (similar to the valour castle+its basement structure).
Within the Dungeon:
1. Consist of narrow paths that only allow 4-6 people to walk through as the same time. This is:
a) To avoid bypass of trash mobs;
b) To avoid a group of players to swarm their way through the dungeon;
2. Traps within the Dungeon,
e.g Hidden transportation floor(transport player's group back to certain point of the instance, while it looks the same as normal floor next to it except there is a slight different);
Land mines (Deal massive amount of damage to players standing close to it when it is activated, also activated by walking on that part of the floor) etc.
3. Mutiple floors, castle structure consist of 5 floors above the ground and 5 floors below the ground is a general idea. While players can decide what quests or bosses they want to complete in there, they can choose either questing the floor above the ground or entering the underground structure.
4. Teleportation into the structure by NPC outside its front door: A "dungeon group" of at least 4 people within a certain level range are needed in-order for the NPC to teleport them into the starting area of the dungeon, while theres a group inside either the upper or lower part of the dungeon, that part of the dungeon wont be opened for other groups to enter. However, a queue will be up for groups that want to enter the dungeon after current group finishes up.
5. Chests with random items guarded by monsters can be found in the dungeon, however sometimes they may be traps (poison....explosion etc.)
6. Some bosses wont be on their own, they will summon slimes/spiders/skeletons like players can do normally, or they will have guardians beside them.
*IMPORTANT POINTS:
1. A time limit will be applied for a group after they enter the dungeon. Thus there wont be a group who entered the dungeon and stay to farm items.
2. No group can enter the dungeon with a character lower / higher than the level limit even the leader / the person who talks to the doorman NPC is within this level range.
3. No teleportation/portal/summoning spells will be allowed in the dungeon.
4. All monsters wont respawn in the dungeon until the time limit is over (which means that the current is out the dungeon already, to avoid farming of certain bosses or monsters for items).
5. Dungeon quest items and the rewards from those quests are not tradeable.
6. Dungeon monsters wont level up.
7. When a player/players is/are being killed in the dungeon, he/they will respawn in a "safe room" (wont aggro any monsters) closest to the place they die; note that this room will not be ahead of where they die to avoid players to bypass a group of monsters.
8. Bosses difficulties are growing exponentially throughout the dungeon, with bosses few times harder than the last one.
9. Bosses in the dungeon will drop at least one godly item + a few lost artifacts + a few uniques per kill.
10. Uniques, lost artifacts and godly items will not be given to players directly in the dungeon, players need to talk to NPC outside the exit of the dungeon to retrieve those items. Whenever an item is being retrieved from the NPC, a player can choose either (want) or (pass) an item by clicking on a pop up screen. If more than 1 person clicked want, the system will automatically transfer it to either one of those person. A person in a dungeon group will never receive number of items >25% of the total items dropped ( in a case of 10 items while 25% = 2.5, the items limit for each person will be increased to 3). All the unwanted items will be randomly distributed by the system to group members through mail. "Dungeon group" will not be able to break apart until everyone in that group are offline.
*These rules are to protect players from ninja/thief who walks away with all the good items and open a new account to enjoy them.
Suggestion about the inplantation of dungeons:
According to the number of people usually in game at the moment, I suggest that 2 dungeons of this type can be invented into the game with level range: 1. Lowest level: "lv 40-->80"; Low level: "lv 81-->120"; Medium level: "lv 121-->160"; High level:"lv 161-->2xx".
As each dungeon is consisted of two parts, so the 4 level ranges will be seperated between them.
The idea is: for example, for the lowest level dungeon: monsters will be mostly killable by 2 level 80 players, 3 level 60 players and 4 level 40 players.
As bosses within the dungeon will give amount of exp according to player's exp required for next level
e.g. 20% of 50000 = 10000 for a player who needs 50000 exp for next level.
This prevents players at maximum level limit of the dungeon to constantly re-entering it and play it as an easy mode to farm items.
godly items as usual are "citizen" only items.
these items dropped with respect to different level dungeons are as follow:
1. Lowest: For level 45-85 players
2. Low: For level 90-125 players
3. Medium: For level 130-165 players
4. High: For level 170-MAX players
godly items will not be able to drop on floor by any reason, not even death.
godly items are all indestructable.
godly items only have positive preffix/suffix on them.
godly items are not stealable by pickpocket.
There are at least 2 godly items for each type of weapon/armor:
(fast + lower dmg / slow + higher dmg)
(Mages cloth armor/Warriors metal armor)
---------------OK...I will stop at here, will add in further ideas later 
