Vertical Polytonality
Here's something I picked up from Yusef Lateef's Repository: stacking chords where each voice is from a different scale. Interesting results -- check it out.
The idea is simply to play a scale and harmonize it by laying another scale on top of it. For example, here's the C major scale with E marmonic minor on top:
Or you could harmonize a fourth apart, as I've done here with C major and F Phrygian:
Here's an example using three scales stacking in thirds, like Lateef does -- this time C major, E Neapolitan and G major:
...or you could even build seventh chords, as in this example with C major, E Harmonic Major, G Dorian #4 and finally B Locrian, which of course means the sevenths match the diatonic ones:
These all, to my ears, produce interesting harmonic motions that you can't get any other way. Finally, here's a crazy idea that I haven't got time to get into just now. I've put C major pentatonic with E whole-tone, which of course have different numebrs of notes -- it will take six octaves (I think) before it repeats!
There's a world of sounds here, and if you know the scales it's quite possible to build these ideas improvisationally, especially if you know them vertically. That's a good test of your fretboard knowledge, too.