First, before buying anything, decide what it is you like to photograph
If you're into portrait photography then umbrella lights might be a good investment. Of course that is assuming you know how to use them.
If photographing flowers and insects is what you enjoy then perhaps a true macro lens would be a good purchase. Don't be mislead by zoom lenses with "macro" in their description. They are really close-focusing. A true macro lens is a single focal length and is designed to perform best at close distances, has minimal light fall-off from center to edge and maximum sharpness from center to edge. Plus, a true macro gives you a 1:1 (life size) image. Close-focusing zooms seldom exceed a 1:4 (1/4 life size) image.
If you're into sports photography then a long zoom, perhaps 100-400mm, would be the obvious choice. Of course, such a lens can also be used for nature/wildlife photography.
IMO, you should fully exhaust the capabilities of the lens you own and determine what type of photography you do the most before rushing out to buy anything.
For now, settle for a good UV/Haze filter and a good circular polarizer for the lens you own. Heliopan, Singh-Ray, B+W, Hoya and Tiffen are all good brands. The UV filter will protect the front element of your lens. The circular polarizer can be used to darken a blue sky and remove glare/reflections from snow, glass, water and painted metal - but not polished metal.
Add a lens cleaning cloth to your shopping list.
