I think it would be really cool if there was a weekly quest/community event that occurred in game, and it could have multiple objectives.
This is just an example, with made up numbers (so adjustments and testing would need to be made)
Kill: 100,000 Fallen
10,000 Fallen Captains
50,000 Hive
PvP: 15000 Hunters
15000 Warlocks
Gather: 10000 Motes of light
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Those are some examples, and everyone who is playing the game that week (Id say segregated by platform) would contribute to the overall weekly community quest. And Each week the options would change, and hopefully make adjustments based off of the previous week's stats. So (in the example above) if 500,000 Fallen were killed, perhaps the next time that option came up on the quest it would be increased from 100,000 to 200,000 or something along those lines. (Just a thought)
For users participation, they would be rewarded based on their overall contribution and ranked, just like in public events.
Anyway, this is just a thought, thanks for your time and consideration. :)
Take care. :)
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Kinda like Mass Effect 3?