What do you not like about your existing lens? Something with a more limited zoom range may be sharper and faster and have less distortion at the same focal length, a prime lens (single focal length) even more so. But of course you'd need multiple lenses to cover the same range. What focal lengths do you take most of your pictures at? Are you using the wide end or the long end most? Are your photos sharp enough? Do you feel you can't use a fast enough sthutter speed because the lens doesn't have a wide enough aperture? Would you like to experiment with shallower depth of field?