Looks like I misread your post. You have a very good lineup of lenses. If you really do want to get a new lens then the 70-300 will give you more reach than you already have (I have this one and love it so far, VR works excellent), the 18-200 covers pretty much what you have, but is a one lens things, although it doesn't go as wide as lenses you have (your f/2.8 lenses are excellent, the 18-200 will only go as wide as f/3.5 and thats at the 18mm end, its small the longer you go), and then another option is a SWA (Super Wide Angle) like Sigma's 10-20mm and Tokina's 12-24mm (IMO, Nikon's is not twice as good as either of these, but it is twice the price). Sigma also makes a 50-500mm lens that looks interesting.
Good luck.