An 18-200m f2.8-5.6 lense???

Hmm? The 70-200 has a conservative 1:3 zoom ratio, the 18-200 has about a 1:11 zoom ratio. They end up at the same 200mm but get there from very different places. The 18-200 has always been recognized as having considerable distortion, often complex, changing as one zoomed. Having a wider maximum f/stop would likely increase distortion as well as the size of the lens, though it's certainly possible (=likely) that the Nikon lens designers know a lot more about this topic than I do, so maybe in a few years we'll see something like what you describe. It wasn't long ago that people thought that an 18-200 with decent quality was impossible.

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--Bob