I always liked idea of open worlds, but it wont work on Fo2.
Instead I would like to have some special event, where for example 60 players with different professions (crafters will be usefull), will be taken on some small Island which was a big town. It might be 4 big maps connected via grids, so no any WM there, u are starting with nothing and can collect stuff by scavening resources like from ecounters, by searching lockers and junks, ofc there should be presented critters like animals and humans. The point would be searching for some Data vaules or anything, if more u gather and bring at end of event, the better reward u get.
The point is that u are closed from other world and naked, need to gather some stuff (It should be done in way, that tier 1 and 2 will be usefull, at least at start, and t3 will be rare). Maybe there cooldowns would be a bit highger to balance healing.
Something like Hungergame, but for longer (event might work for 1 hour, 1,5 hour, 2 hours), u can replicate there if u want so after die u still can get something. And instead of spawning alot of drugs/weapons/ammos, there would be spawning mostly resources, so this event would create such great post apocalyptic theme.
It would be more like Battleroyal where u dont have to be lastman standing like Hunger games, but U have to collect some resources.