For figuring out fun harmonies in other tunings, something I do is make a fretboard chart with every note written out. Then I'll circle the notes belonging to the I chord in a color, the notes of the IV chord in a different color, etc. You can do a similar thing but for suspended chords, 7ths, etc.
I like Muriel Anderson's book
https://www.amazon.com/Building-Guit...GK7MYZGDE2Q24Q
Mark Hanson and Howard Morgen also have good fingerstyle arranging books.
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