A few ideas for possible additions to the game, some of these will probably have already been mentioned.
Squad games would be a huge addition in my opinion, a few people have mentioned seeing guys who teamed up in game, so a duo mode and solo mode would fix that issue.
Weapon customisation, such as sights and silencers would be a cool addition.
Traders to buy/sell loot would be cool, traders could also have quests to add something different to raids.
I'll post more ideas in this section if I come up with anything else