A few suggestions (even if some were already said):
- 4 sensors / arrow, arcade layout due to the fact that it is a tried / true method with most dance simulation arcade games.
- X / O buttons separate from the U/D/L/R arrows in a separate control box
- Metal bar [DDR OR PIU/ITG style] with cushion, attached similar to how a real DDR pad's bar is since the way of adding a metal bar feature that most manufacturers use leads to a really wobbly bar.
- Corner brackets being like those in the arcade. None of those giant brackets with 4 screws that some manufacturers use - that just makes it harder to play well.
MOST IMPORTANTLY: - Parts always being available so one can fix the pad *IF* it becomes necessary