The fretted harp!
12 strings exist since quite some decades. On those every string has a second one, an octave higher, except string 1 and 2 which are simply doubled. They have a very rich sound, but playing them is not different than usual ones.
Then came 7 strings, 8 string and 9 strings to add more scale. Also 5 string, 6 string and 7 string bass guitars aren't that uncommon anymore.
So, being Stringray, I gotta find some more strings!
The idea is simple: attach a fretboard to a harp.
But how can one play that if not standing in fron of it, with your back to the audiene, constantly movin up and down and showing them your fat ass?
So, you still had to play it like the harpist does, from both sides. For achieving this, a normal fretboard is imposible. But there's still a way. At first, -yes- double the amount of strings. Every string must have its twin, sitting behind it. In between the string layers would be steel bars that carry the steel frets, one on each side of the bar, so each of the twin strings can have frets.
good playability, a pretty cool looking artist, and --- MORE SRTINGS!
2. Tonal set up:
The harp is constructed to transport the strings' frequencies to the sound body on both ends, so it's quite tricky.
Now attaching frets brings of course the 'problem' that it will always produce 2 notes, one on the upper and one on the lower end of the string.
There's only one way to gain control over this. The fret distances must be set in a way that a harmonic is always given. Both ends of a string must be in harmony in every given scenario.
That also gives options. Since there are many strings in similar tonal range, you can have different string set ups for different dual harmonic scales.
Advantage: nobody will ever be able to play this.
I swear, if you ever build that, I will not die until I've learned to play it!