Try using the Jazz theory of playing scales in relation to chords. So if your playing over a Gmaj - Cmaj - Dmaj, harmonize it with G major, C lydian, and D mixolydian. Now just concern yourself for now with the simple structure of the chord, don't worry about 7th's and 9th or whatever. Take the first chord G major, made up of G, B, and D.

G major - G, B, D

Now fill in the rest of the notes.

G major - G, A, B, C, D, E, F, G

Now alter them in any way you like, flatten or sharp random notes. Try recording a loop of a G major chord and play it back and mess with and experiment with each note over the loop. Find the harmonies that you are looking for because only you know what your looking for. You may come up with something like.

G major - G, Ab, B, C#, D, Eb, F, G

it's even cool to have 2 notes the same like.

G major - G, Ab, Bb, B, C#, D, Eb, F, G

Whatever you come up with will be the sound your looking for. Then do the same for a minor chord. Now about extensions of chords like 7th's and 9th. Just alter the appropriate note to fit the chord.

Say you have a G major 7th chord. Just alter your scale like this, so the 7th becomes a major seventh.

G major 7th - G, Ab, B, C#, D, Eb, F# (instead of F), G

Hopefully that makes sense and it helps you find the sound your looking for.