I think a sniper campaign is a great idea. But a couple of suggestions.
Why work for the US? Its so cliche... How about that you choose from like 2 or 3 different characters of various countries.
And ive got a suggestion for where the campaign could be. In Bosnia in 93' there were many Serb snipers, and they were killing off loads of ppl. Even 4 year old kids crossing the road.
Look up the infamous 'sniper alley'. You could make a scenario very similar to this, in a country of choice. You could, say, be a NATO sniper, who has to neutralize these snipers. It would involve you reconing the area, scouting for snipers. Then you would have to eliminate them yourself. You could also call in artilary to bombard sniper positions in mountains or whatever. You could also be ordered to assasinate the the highest ranking enemy officer in the region, stuff like that. (damn... if i wasnt working on sumthing else id do this myself.)