Since you already have the 18-200 as your "general purpose" lens... it has covered most of your shooting range... so now... you can look at fast lenses or ultra wides... here are some suggesstions with budget in mind:
Tokina 12-24 f/4 - Ultra wide for landscapes and taking large groups, etc. [as compared to Nikon's 12-24 f/4] - about $700 new
Tamron 17-50 f/2.8 - Fast lens for that range [as compared to Nikon's 17-55 f/2.8] - about $650 new
Nikon 80-200 f/2.8D - Fast telephoto lens but look for the one-touch AF-D or two-touch AF-D models... which are more affordable now 2nd hand - about $700 (1-touch) / $1000 (2-touch) 2nd hand
Then here are some prime lenses for portraits. The range would depend on what kind of shooting distance you are looking for.
Nikon 35mm f/2D - about $500 new
Nikon 50mm f/1.8D - about $180 new
Nikon 50mm f/1.4D - about $480 new
Nikon 85mm f/1.8D - about $620 new
What you can also do is to look through all the photos you have shot with yoru 18-200mm lens. Look at what range you normally shoot at, e.g. I normally shoot at 35-70mm for portraits, and 17mm for landscape, etc. This would give you an idea what lens would complement the 18-200 which you have.