WLD, that should be true for any arranger. Once you've got the hang of some of the common altered tunings (eg. drop-D, double-drop-D, open-G, open-D, DADGAD) and you've figured out what kind of distinctive sounds and movements you can get out of those tunings, you should be in a position to invent your own tunings to meet the needs of a particular tune.

I don't think Carthy is particularly setting the way in trying different tunings. For one, Davey Graham invented DADGAD and experimented with all sorts of tunings too, so 60s musicians like Carthy already had an example of someone doing it. And really, if you've got any sort of bump of curiosity about you at all, not long after you first drop the low E to D the thought should occur to you: "I wonder what happens if you change other strings?" The only difficulty is first getting the confidence to break out of standard tuning - after that, the rest follows logically.

Thing is, any guitar tuning is always a compromise. Open drone notes can fill in the background beautifully whilst freeing you to move up the fretboard, but if you hit a chord that doesn't match an open drone note then you need to find a fretted note to match. Strings tuned close together (eg. the upper GAD of DADGAD) make it easy to play legato harp-like passages by alternating strings, but there's the disadvantage of reducing your range. Conversely expanding the instrument's range (eg. dropping low E to D) means that fretting some notes on that string changes the notes available to your hand on other strings, which can be an advantage or a disadvantage depending on how it works out. Standard tuning didn't become standard because it's particularly good at any one thing (although it's usually a good choice for playing in E or A) but because it's the least-worst at everything.

Bryn, I think your tuning is Cadd11. It's not a Csus4 because you have the Es in there which make it major, and it's not a C11 because you're missing the 7th.

Graham.