Ake - it was probably me making something over-complex by explaining it badly!

If you are wanting to transfer 'known' things then open tunings might not be the thing. (But on most of them an E or A minor shape usually works! Check Joni Mitchell who plays full barre chords a lot and Am7 shapes a lot and the tunings take over. If you look at about 65 seconds in to this video you'll get an example of other slightly non standard things she does!)

I reckon that DADGAD would suit Scottish tunes with the thumb picking out the tune and using the top strings as you described. It would work well. The sound would fit some tunes well.

I go through phases of using different tunings. Sometimes if I get stuck in a rut I experiment. A friend played Guinevere by Crosby Stills and Nash a while back and once I came across the tuning EBDGAD it just works. And it's then quite fun to experiment from there.

Here's a little quick video that i just did on my camera of a tune that starts in C and ends in D but I have never worked out what the chords etc actually are it is just what it is. As it is in an odd tuning pretty much none of the chord shapes that I play did I know before I started noodling so it is all just experiment and what sounded okish to my ears - and I don't recognise the chord shapes as any that I use in pretty much anything else. I haven't played it for years so excuse the mistakes. In fact I was chatting to my son about it the other day and said that I could not consciously remember it at all apart from what the tuning was. But I knew if I could find a part of it that I remembered the rest would come back as I played it. And it did - but it has sat in my head/fingers without being played for quite a few years.

It links in with your original post as to whether NOT doing things makes them better. I think on balance not as it is not very practised or accomplished because of lack of practice or familiarity. But it does demonstrate how amazing brains and muscle memory are and what they store and can recreate. I genuinely could only remember the first four notes and a rough idea of where the bridge/chorus bit was on the guitar before I started letting my fingers and brain find it without my involvement! I believe that when Glenn Campbell was fairly late on in his dementia path he could still remember his music and songs. Weird things brains.