I know that you're releasing the summer event, so this is bad timing on my part, but...
I'd love to have world events where the whole community has to come together to solve a problem and is rewarded for how much effort they've put into it.
For instance, a strange meteor has fallen from the sky and is releasing a caustic gas. The community needs to uses pickaxes to break it away before all humanity is killed. Maybe this meteor will randomly spawn creatures that can be killed for a weapon upgrade reward for 24 hours or a 2%-5% permanent upgrade damage bonus. (Like spawn demented developers who attack us for a little revenge and we have to fight them back to their planet--we all know a zombie L3fty or body-snatched Anakiro might like to beat the crap out of some of us). If the community succeeds at the end of the week, they're rewarded with awesomeness. If they fail, I guess nothing lol.
That's a crappy example but without stealing ideas from other games, it was the best I could come up with off the top of my head. One game had a giant cake that the whole community had to eat. It took a week or two to actually eat the whole thing. In the end, the whole world was awarded double experience for a weekend and those that did the most work, were rewarded with extra prizes.
This would differ from seasonal events in that the whole community has to work together and not necessarily in groups, just side by side, working together. It wouldn't be like the hulking troll where whoever hits first gets the reward. This is a major world event. No tank, no healer needed, no groups needed. And all levels are invited to participate.