The Call Of Duty game (Black Ops) has its story revolving around number stations, Mk-ultra brainwashing, black operations and Soviet biochemical weapons. I want to have my story's concept pretty much similar like that with some differences, but I don't want to infringe on the original story; does anyone have any ideas for my story to not be a ripoff, but can give off a similar atmosphere?
First of all, keep in mind that CoD did not invent most of these things. They are either things which actually exist or popular myths. Often a mix of both. They can hardly monopolize these story elements.
So a good start is to do your own research. When you dive down the rabbit hole on your own, you will find that there are a lot of interesting facts and myths around these topics which the CoD developers did not put in their game. And don't just focus on the Soviet Union. There are also some pretty interesting secret projects other states won't confirm or deny they might or might not have done during the cold war. And considering that your game is fiction, and that most of what you are going to find during your research will also be fiction invented by some conspiracy theorists, you can also extrapolate from what you find and invent your own.