Given that most sequencers use a flat velocity rate (by default) over the entire length of a pattern; how about having a physical way of entering the velocity level (how hard that virtual drum is hit, etc.); by tapping on a pad rigged to sense how hard it is hit ?
To give a idea how this could work; a given pattern would be loaded into the sequencer and played, then you tap on pad, to the beat …& the resulting velocity values could be sampled on the beat.
Also The difference in timing between that of the sequencer & the pad taps could be sampled & used to shift the timing of the sequencer to give a more natural or excentric sense of rhythm.
This Could provide a way of making the sequencer sound very organic & could be immense fun ! …and of course the ‘velocity data’ adquired could be used to set any other perameter you wish …mmmm 
On the hardware side of the implimentation i can think of roughly five ways to do this:
1/ Simply sample the velocity value from a velocity sensitive keyboard.
2/ Use a piezo cell as a velocity sensor (analog circuitary might be a pain - but has probably been implimented already for diy drums)
3/ use a flexable steel strip as a ‘velocity pad’ & and use a variable resistor + leaver assembly (see diagram below). (getting good range of velocity values to sample might be awkward, switch probably needed to give a accurate rhythm reference)
4/ (as in 3) Use a coil & magnet assembly + opamp & charge storage circuit.
5/ (as in 3) use a magnet & hall effect device ?
Oh.. and i read somewhere else on the board of 'force sensitive resistors 
Also another useful thing to impliment, would be a ‘normalize’ function; whereby the highest velocity value sampled is raised to the max value & then the other samples are raised with respect to this.
What Think to this idea ?