So stuff like "classify ground" "classify vegetation" etc would be great! Ground is the main one.
And also, like I posted about before. To be able to classify model keypoints from the ground class, so you get a small amount of points that accurately defines the shape of the terrain.
There are currently no free software that does this, and it would be a serious advantage.
This is the ground classification window in terrascan:
And this is the model keypoint window:
I don't know if they are based on open algorithms, or something Terrasolid made from scratch, so I don't know how hard it would be to implement.
But there is a serious lack of good software to classify pointclouds, that don't cost a fortune :)