I sadly haven't seen a small scale successful group event that wasn't exploitable or just so camped some never got to participate.
What I have seen work was the events like "Plagues of Despair" or the World of Warcraft "Ahn'Qiraj" event. Specifically gatheirng the war supplies
http://wowwiki.wikia.com/Gates_of_Ahn'Qiraj
Basically global kill, collect, escort quests that everyone could participate in solo or as a group. You could work on parts of it solo and for others you needed full groups to deal with the monsters or puzzles.
So some of the monsters could be champ spawn boss level and would require people to access them through a series of group puzzles.
Others are more like the peerless bosses which require key farming.
Others are just straight kill and loot.
And then there is the resources gathering component which is 100k+ (depends on how active shard it may be millions)
Lastly there are Admin run events like a monster invasion of Vesper which the players have to fight off waves of monsters or the town becomes lost. A series of these in different locations. Like if yew isn't defended we could no longer use it as a town. Then in later events the Players could try retake it. thus creating a second event of players attacking and Admin's Defending.
Reward for something like this is
Participation: Deco stuff, tokens
Completion: Access to new areas
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But my favorite quest has actually been used on a few other shards
Assassination.
As a PVP event.
Every participant is given a book with the name of another participant in it. Then everyone is released into a small area. You could only kill your own target and defend against attack.Dealing damage to someone not your assassin or target wasn't possible/disqualified.
You then loot your new target from your target until only one is left.
As a PVE event.
This was run through an NPC guild. You would collect your "target note" from a drop box in the guild building. Then head out and find them. One of a few special weapons had to be used. you gained access to them by gaining rank in the guild. Once you completed your target task you would be sent to some other out of the way place in the world to collect the next one. you could do 5 a day and they got progressively harder. Each was also storied and time sensitive, getting progressively harder during the chain and as you rank up.
The instructions either dymaically updated or you got new ones as certain stages.
Sample mission chains:
1. Steal a prison key from Guard Captain Althen in Yew.
2. Meet Jimmy the snitch in Yew crypts
3. Talk to Edgar Sootfinger about where he the diamond (this required sneaking into the prison and using the key from 1)
4. Travel to location. i.e Hyloth Balron Room and find correct chest.
5. Deliver the Diamond to Thiefmaster in Bucs Den.
Sample Mission Chain.
1. Meet Edgar Sootfinger at The ratmen camp in Ice dungeon. (would need to find a invisible character Tracking and method to reveal)
2. Deal to the guard by poisoning their Stew in the Orc Dungeon (poisoning skill needed)
3. Steal a uniform
4. Infiltrate Lord Valender's home by using the uniform
5. Retrieve Lord Valender's ring (Steal it from NPC or Assassinate him and loot it)
6. Hand in to Edgar.
So targets are in Guard Zones, all facets, in dungeons.