I'd be interested in a PIU (separate) pad as well as a DDR/ITG pad.

Especially the PIU pad, since there are very few options for that game compared to DDR/ITG.
In all cases, it's extremely important for the layout and feel of the buttons to be faithful to the arcade. Should there be an optional bar or a mandatory bar? (I play no-bar on all games either way, so it doesn't matter to me, but I'm sure there's enough players who could benefit.)
In PIU's case, it's also best to have the pad be compatible with PS2 and PC, since those are the primary platforms for home PIU.