A good lens for an aspiring photographer?

I currently have a Nikon D50 with an 18-200mm lens.
What would be a good lens to get in addition to this?

That depends on what you want to take photos of. Different lenses are used for different types of photography. You do understand that right?

The 18-200mm is an excellent walking around lens. Maybe the next thing to add to your collection would be the Nikkor 35mm f/1.8.It will give you low light capability at a very reasonable price, plus it is an excellent lens. This is equivalent, for your D50, to the "nifty fifty".

Always good to have a 55mm lens.

If you can afford it, get a 50mm f1.4 That is a pro level lens at a reasonable price. With that lens you can takes some beautiful pictures.

I would suggest the 10-24 mm wide angle lens so you can take brilliant landscapes

Or, you can buy a nice 85 mm macro lens if you like to shoot closeups

17-55mm 2.8. The aperture will give you great bokeh.