The biggest issue with existing whiteboards is that they over-simplify the path of writing, which when using a stylus and writing quickly an "n" or a "u" both tend to get turned into something like an "s" rotated by π/2. This can easily turn terrible student (or professor) handwriting into something far worse. Obviously this issue alone isn't worth a full rewrite... none of this is worth it to me, because I don't have time.
The other major feature I'd love would be to support LaTeX math through typing, for students who don't have a stylus device. I've not found a whiteboard that supports this, but it shouldn't be too hard with mathjax.
Let me know if you think this sounds like an interesting project, and I can share ideas. I expect this Fall that many people could really benefit from a decent whiteboard.