The tweak/sculpt tool is nice, but sometimes it seems to introduce a little too much distortion. It's also quite slow, and setting the appropriate width can be a bit annoying. How about implementing something like this? http://www-ui.is.s.u-tokyo.ac.jp/~takeo/research/rigid/curve/index.html
The two big ideas in there are as far as I know: 1) automatically setting region of influence based on maximum amount dragged. 2) angle preserving deformation -- for instance if you try Inkscape's sculpt tool on a crown shape like in the lower right corner of the applet there it's pretty impossible not to turn the straight lines into curves.
The smoothing thing in that demo isn't bad either, I don't know of any way to "brush on" smoothing in Inkscape right now. Maybe the tweak/sculpt tool can do it somehow though?
A couple of other line drawing enhancements that I think would be nice are * a mode where scribbling back and forth over a line erases it. * a mode where drawing over top of a portion of an existing curve would automatically splice in the new part.
I've seen these latter two techniques used in some of Igarashi's papers, and they look handy. The first is used in Teddy. I'm not sure where I've seen the second.
I think this could maybe even be a nice little set of mini-projects for a GSoC.
--bb