If you wanted a wider field of view, your 18-200mm f/3.5-5.6 VR lens have the 18mm that is wider than another of the mentioned lens thus far. When you go wide, the bokeh reduces. Bokeh at FX 24mm and below is not quite that distinct.
Any wider than 18mm DX, you'll have to look at the Tokina 11-16mm f/2.8 II. This is a DX lens too.
Unfortunately, I doubt it is possible to have both smooth creamy bokeh and wide angle at the same time unless shooting at very close distances which depending on the distance, can cause distortion.
I'll list out the options for you
1) Tokina 11-16mm f/2.8 II :- For landscape, indoor landscape and some subject isolation at the 16mm end at close distance
2) Sigma / Tamron 17-50mm f/2.8 :- 1mm wider and same coverage as your 18-200mm f/3.5-5.6 VR lens but with a constant aperture of f/2.8 that is more capable for contrast detection in low light and subject isolation at the longer ends)
3) Sigma 18-35mm f/1.8 DC :- Overlap coverage on what you have except that it has a max aperture of f/1.8 that is great for low light and subject isolation
If budget is not an issue and don't mind under utilizing the lenses, the following lenses could be worth looking at:
1) Nikon 24mm f/1.4G (Gives equivalent FoV of 35mm on the D7100) It's an very expensive prime lens. I think its the most expensive wide angle prime produced by Nikon
2) Nikon 14-24mm f/2.8G (The widest FX lens produced by Nikon, gives an equivalent FoV of 21-35mm). Its a very expensive Wide angle zoom lens
Following photo taken with the 14-24mm f/2.8G at 24mm, f/2.8.
I have actually sold off my 35mm f1.8g as I was hoping to get a wider field of view with the 28mm f1.8g...but not very
sure if the field of view is appreciably wider and thus am looking also at the new Sigma 18-35mm f1.8g which is not available
on Nikon mount yet....