hehe, you know what I think... 5-key and 10-key beatmania with MixWaver II is awesome. To get 7-key or 14-key song support, you have to build your panel pretty special.
I bet you could do "poor man's DDR" by putting large, widely spaced buttons on the panel instead of the dance pad. I don't know how you'd go about making a drum-mania or a guitar-freaks game work on a cab.
Got any favorite songs in beatmania yet? I'm trying to get where I can play the middle section in "Blue Water Blue Sky".
I also suggest "Bourbon Cowboy", "Plasmasphere", "BrownWater", and the "Dragnet Remix"