Vertical slides... very interesting...
Since I'll be having tons of free time during my internship this summer, I'm planning on completely rewritting Beats (new version will be called "Beats Portable" and be hosted at
http://code.google.com/p/beatsportable/ ), but this time, fully open source and under the Modified BSD License and probably also with a desktop port for fun.
Before its release, I'll probably continue modifying the current version of Beats to include experimental new features, but I won't be working on the engine since Beats Portable's engine will be very different. Particularly, I'll be writing it with slides, holds, stops, BPM changes, and 4+ buttons in mind (most likely 8 since Beatmania is 7-key plus scratch). If Tap Sonic is just like Beatmania gameplay-wise but with vertical slides,that should be possible if I implement osu slides support flexibly...
All just speculation and musing right now ; )
~Keripo