The probability parameter is great for adding variety to a pattern but it would be cool if random velocity was available as well.
I imagine it would work as follows.. 
A random parameter is added to the step edit screen (with the velocity, note and probability settings) so that each step (and substep) can have a random setting. 
When a step is played, the actual velocity is calculated like a multiply.  So a random setting of zero means the output velocity is the same as the velocity setting.  A random setting at 127 means the velocity is completely random between 0 and the velocity setting.
It would allow adding a little random velocity to patterns which sounds more natural.  And added benefit is that it would allow some cool random modulation when using velocity for other modulation destinations.
                 
        
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a|x
So i'm not sure if adding 4 (random, note, vel, prob) new parameters to each step is feasible.
the other values are 7 bit if i am correct (because of midi cc range) so maybe it might be a idea to have the data coded somehow to save memory?
i think oliviere wrote something about that topic recently over at mi forum, but i cant find the thread right now.
- a random parameter set on a per track instead of a per step basis
- reduce probability resolution to 4 bit, add 4 bit random parameter = 16 steps each + a global vol,note multiplier set per track