Couple of other thoughts.

If you want to get a fantastic book on harmonising tunes with a load of theory and a load of useful info get hold of Eric Roche's book 'The Acoustic Guitar Bible'.

In one part he takes the tune 'Twinkle Twinkle Little Star' and harmonises it in very different ways - from the expected 'right' way to a very, very different version with passing chords, suspensions, inversions etc thrown in. Most useful thing on the harmonising of tunes I have ever come across.

The other person you might listen to is someone like Denny Bartlett from Last Night's Fun who accompanies mostly in DADGAD. I learnt a lot from watching and chatting to him as his approach was very different to my 'playing the right chord' approach that I had up to then. Both of the above have added a greater degree of freedom to my playing and thinking.