If you're thinking communal then really two options;
You can explore together - whether this in the overworld, nether or end doesn't really matter. Though with the end, for example, a group of us could kill the dragon and go end raiding.
The other, arguably more common communal projects are builds of some sort like the railway project that is still in progress. The railway build though is fragmented due to it being a railway so once it leaves spawn it is everyone on their own. However, we could go for building something communal where we all contribute in the same area making it everyones. There is the obvious village or castle and surround villages but I've seen cool builds of upside down houses built to the roof of caves and overhangs, massive underground caves dug out and built in or under the sea cities etc. The limit is our imagination though it would need to a be communal idea / decision I think so people feel motivated to help out the build.
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