If your main goal is more reach than the Nikon 18-200mm, the Tamron 18-270mm is a reasonable superzoom option; community feedback suggests it performs decently for this class of lens, though superzooms always involve compromises in image quality and maximum aperture.

Other options mentioned:

• Nikon 28-300mm VR: gives more reach, but starts much less wide than 18mm, so it changes how useful it is as an all-in-one lens.
• Tokina 16.5-135mm: likely the 16-xxx lens you saw, but it does not help if you want more than 200mm.
• Nikon 70-300mm VR: a strong alternative if reach matters more than having one lens do everything.

One useful point: at close focus distances, the Nikon 18-200mm’s internal focusing can make its effective focal length shorter than a true 200mm. So depending on what you shoot, a dedicated telephoto zoom may give you more real-world reach than you expect.

In short: for a single-lens solution, the Tamron 18-270mm is worth considering; for better telephoto usefulness, a 70-300mm-style lens may be the better choice.