[The Fifth Horseman]

Okay.

Is that CT group the player fights in one that exists in real world or a fictional one?
Who are the terrorists? What is the threat they pose?
If you have no idea what to use for the latter, how about changing the location to a nuclear power plant? These things can be pretty dangerous in the wrong hands, so the player would be trying to prevent the place from becoming a second Chernobyl.

You do, of course, realize that sending a single man into a bunker (or whatever) full of bad guys isn't logical. Either the player will be aided by some AI-controlled bots or you'll have to come up with an explanation of what happened to the rest of the team.

Also, does the engine allow the player to backtrack to previously visited levels? If it does, it would be possible to have the player backtracking to previously visited locations but with new objectives present (ie finding and defusing several explosive charges in the building within a set time limit)