i love the idea of a communal adventure map. the problem is a lot aren't designed for massive groups, so im just not sure something like that could work. one idea i thought of is maybe putting up a complete the monument map, since it plays closely to regular minecraft compared to something more structured. such maps wont really break if someone new joins midway, they can take a decent amount of time to complete (good for a nice weekend event for example), can easily host lots of people at a time (some items might get scarce but it would balance out with the theoretical amount of infrastructure that would be established by players for basic needs), and play to many strengths of varying players (combat, building, creativity). it would have something for everyone that plays on nerd, and you could easily open it to say 20 or 30 slots no problem without crowding the map (obviously some ctm maps are smaller than others, but generally speaking). the only real downside is it could be a bit on the easy side with so many people, but thats only if everyone decides to work towards the same objective at once.