Re: Advice on harmonizing (not lame 3rds and 5ths)
It is a difficult question, but that's how I see it...
If you try to stuck 4ths to create new chords, 3rds will appear again as 10ths, so it's not as cool as it seems. Ex (C chord in 4ths)
C - F - B - E - A : C (tonic), F (fourth), B (major 7th), E (major 10th, that is, major third) and A (major 6th) This have the same notes as a CMaj13 if you respell the 4th as a 11th and the 6th as a 13th.
I suppose you don't want to use diatonic notes (like the common harmonization in 3ds). In that case, imagine you create a melody which starts from fret 5. Record it, and now play over that the same melody, but now starting on the Major2th (freet 7). It will sound strange but good. I heard a piece of music which included pairs of instruments doing this technique with different intervals (ej b7, #4, 5th).
Each interval makes the melodies sound very different. Depending on the interval it will be easier or more difficult to harmonize. That is, a melody in CMaj played a 2nd maj away, will produce F# and C# -just add a whole tone to each note- it's like if you were using a major scale with an added b2(C#) and a #4(F#), that is: C-C#-D-E-F-F#-G-A-B. You could add that notes in the chords which CMaj produce.
Sorry if it's a bit messy, but that's how I see it...