You can all be in the same area(Such as Inside a house or the wasteland) but if you want too go into a building or go into a area that would need too load the other players would need to also accept, If one dies and you have a doctors bag or stimpack he can be revived but maybe in hardcoe he just dies, if one is talking the other plays get svcked into the chat and can't more unless it is something such as taking a quest or buying things, Also drop in drop out co-op, I also would think that all quest items will be given too all like in borderlands only they can't give someone an item they got from a quest but can still sell it too a trader for cash, There would most likely also be trading from player too player, if they feel like they have room for it they can make a pvp area like in Borderlands one in which players are given certain weapons too duel each other with no consequences, and finally waiting and fast travel would be a group vote but if they want too the host can set it so he makes all the decisions... Now Bethesda these are some ideas on how multiplayer co-op would work, I know you probably wouldn't even make co-op and if you did would have all the rules and mechanics would ass-backwards compared to these but here you go for the lol's! If you have any ways on how it would be better reply!