Jonathan already said it, but night shots with the Milky Wayin the back ground would be great. That means a fast prime in the 24mm range. Might be worth while doing a two shot composite at night.

Otherwise a TSE. Given your current set up, the 17mm seems a good choice.

I did not get a 70-200 for a long time, but since going ff I find myself using that for a lot of landscapes... including waterfalls. I use it way more than I thought I would (when I can get out to take pictures). You can zoom in and isolate some of the interseting areas of the falls. Keep that in mind... sometimes the "wide angle" card is overplayed on waterfalls and everyone's shots end up looking the same.