I would love to see a sit down at the piano with Brian and Van Dyke just chatting and playing some tunes. The recording of them jamming on H&V in the Endless Harmony disc is one of my favorite tracks ever. Listen to it all the time, I can probably recite it verbatim.
I love the bonus interview on the BWPS DVD where Van Dyke interviews Brian for a while. Brian even plays a few 'feels' of some songs on the piano. Good stuff.