3D!
For example if you wanted to look at the particle projection of something in 3D. It will be viewed in 3D too. Check these out: http://url3.tk/exp.png and http://url3.tk/stereo/ This would require adding support for functions returning vectors, or maybe more easily parametric equations of time.
Would this be a waste of time? Any ideas anyone?
Once we have 3d it would be cool to have the ability to plot contours, and a grid texture on the surface (if fast enough)
I think 3D is a very good idea. I think the ability to display a solid surface, in addition to a particle projection, would be pretty important though.
The latest gl2 branch has 3D. :)