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I know City of Heroes use to do this and that game was made way back when? so why don't you see more MMORPGs take a page from COH and games like Diablo 3 or Torchlight 2 and give dungeon runners near inifinite replayability.
Randomized Scaling Dungeons
1-6 players can enter
mobs scale depending on number of players
rare loot chances are raised the more players. so even 1 player if lucky can recieve a rare drop if they down a boss.
i think this would give players of all types tons of fun, from the soloer to the duoer up to the guild groups. I am just surprised more MMORPGs haven't tried a system like this.