FL Studio can take the chords and scales in it and create random melodies out of them. It let's you choose the key, scale (or chord), octave, octave range, length, and several other factors, and makes a random meloody from it. It can randomize velocity and pan, too. I think that's what you're looking for, but it's not a plug-in.
Well I´ve tried the demo of FL studio. As far as I get out of it, it can only generate patterns of chords or scales, that the program has defined and only one bar. It´s like a appeggiator
for techno music. What I look for is more composing tool where you can define your own tones/pattern and then the tool/plug-in generates variations including the same tones but in different order.