Sardog gave good advice.
You might want to shoot around a bit and see which way you want to go next. There are at least four possible directions:
1. Wider. Like 10mm or shorter, for actual wide-angle shots (your 18mm x 1.5 = 27, not really wide angle). Panoramas, waterfalls from up close, interiors. Affordable, easy to use.
2. Macro. Either a dedicated macro lens, or an add-on close-up lens, or macro tubes. Make trade-offs between price, weight/bulk, and convenience, but any of the options will open up a big new world of subjects.
3. Longer. 200mm with a 1.5 crop factor isn't bad, but if you really like shooting birds and other wildlife, you'll crave something even longer, and faster, and with image stabilization. But you'll want a bigger bank account, and you'll need a tripod if you go past, oh, 400mm.
4. Quality. Fixed lenses are less convenient, but higher quality, and relatively affordable. You might also consider trading that wide ranging 55-200 zoom for something like 50-100 plus 100-300 (not sure what the options are for Nikons); usually you get much less distortion and better speed if you're not trying to cover such an extreme length range. But you need to study your potential purchases carefully, you can't judge the quality of a lens solely by its length range or even by its price - though top quality usually does come at a high price.
Myself I carry a 28-100, a 100-400 (sometimes- it's heavy!), and macro tubes. I might add a wide-angle someday but I don't miss it much.
PS If you like a challenge, start reading about flash photography now - there's lots to learn. See
www.strobist.com
