Audio data would be perfect but I realize this is a MIDI app. Maybe note number could be used as frequency modulation and note velocity or aftertouch as amplitude modulation...?? An XY controller with FM on one axis and AM on the other....oooohhhh!
A feature-packed modulation block - for example selectable waveforms, multiple LFOs, being able to link LFO tweeks to another LFO etc - could add to the Amazement Quotient.