Personally, I would eliminate helicopters and the parachute mechanic. maybe replace it with a rappelling ability. If you think about the original story about illegal soldiers engaging in unconventional warfare in a foreign country then helicopters are pretty much out of the question.

Oh I would also limit gear by weight. In hitman, it's pretty amazing just how many apples and coconuts you can hi in your jacket without it being a problem. I would limit space and gear on the person. Like you could cache it but you have to come back to the cache to get it.

I'm not sure the game im describing would be a AAA killer so it may just have to live on in my dreams.