It depends on how many players you plan.
If it is private/whitelisted/low playercount, have a small map with a set plot, sortof like a survival, and how to get past a really big problem.
If public/large playercount, make it a very large map, and make it really explorable. Backstory, but no set plot. Let players do the talking and it'll be more enjoyable.
Its not easy to create plot with lots of players because not everybody will be informed.