I was thinking it could be a good idea to have a lesson on parts of speech to help people understand the lower/upper intermediate lessons. I'm pretty sure most of the Japanese words for parts of speech have been given in lessons that cover a particular grammar point, but never as a group that can be referenced.
What would be especially useful, though, is an explanation of phrases/structures that frequently come up in Natsuko and Co's explanations: 'word X means Y', 'if you translate directly...', 'if you add X to Y, you get meaning Z', and more useful ones that I can't think of. If you consider that a lot of us are now also listening to EnglishPod, I'm sure that most people would find this beneficial. On top of that, it'd also come in handy for explaining English points to Japanese friends.