I spent a while in Mongolia and frankly I think that what you (will) have is fine, with some minor tweaks.

I'd add a good quality circular polarising filter for your kit lens as well (cheap ones have weird colour casts, good ones are colour-neutral at all angles).

For super-wide shots, I was quite happy with stitched 17mm shots. I don't think wide lens is a must. MS has a free stitcher that does a fair job if you make an effort to rotate your camera sensibly close to the lens' nodal point.

Night-time photography is also possible and with the crystal-clear skies I'd heartily recommend it. Watch out for the sharp temperature changes to night-time and the resulting condensation on your equipment. Also bear in mind that your batteries will have a significantly lower charge at low temperatures so it's worth keeping your spares close to body heat. A tripod isn't absolutely necessary if you find convenient resting places for your camera.

Unless you're in Ulaanbataar you might not find convenient charging points, so battery-saving measures make sense (disable review, lower lcd brightness, keep batteries out of camera unless shooting...) and I'd consider packing some spare memory cards too.

If you've got a point+shoot camera, bring it with you - you won't always have the time to whip out your dSLR ;-)

Lastly, I'd strongly recommend lots of practice of the various shots you'll be taking so you can spend more time enjoying the experience and less time setting up shots; having a list to hand of the shots you know you want is also quite helpful.

Oh yes, and expect your gear to get fairly dusty, no matter how much care you take to keep it clean. Dust spots can be removed after the fact fairly easily in any case.