Before starting work on this project, I considered several different isomorphic layouts 

• Harmonic Table Layout
• Wicki-Hayden Layout
• All Fourths Layout
• Janko Layout

Below I show the layouts and major/minor chords of each and my reasons for not choosing them.

Harmonic Table Layout:

Specifications:

• 7 Octaves
• 252 keys : 84 notes -> 3 keys : 1 note

Chords:

[Blue coloured indicate alternative keys to change the type of chord - e.g. Diminished chord by using F# instead of G]

• Hexagonal keys allows for chords to be played with fewer fingers
• All key caps would have to be custom made
• Vertical chord patters did not seem very comfortable to play

This was my initial choice for a layout, but at 252 keys, I very quickly became apparent that the weight of the switches alone would be too much.

There was a commercially available keyboard using this layout called the Axis-64 by C-thru, but it has now been discontinued.

Wicki-Hayden Layout:

[Note columns of sharps on left are repeated on the right]

There are two general configurations:

1. One tall keyboard - with the hands one above the other
2. Two split keyboards - each hand has its own keyboard. 

Chords:

I find these chord shapes really uncomfortable (especially the blue modifiers) - this is the deal breaker for this layout.

All Fourths Layout:

Specifications:

• 150 keys : 50 notes -> 3 keys : 1 note
• 4 Octaves

The All Fourths was a strong contender, but it's few octaves and large number of buttons were against it.

Janko:

[image from wikipedia]

For me the Janko is not suitable for several reasons

• Just as long and as heavy as a regular piano keyboard
• The chord shapes are not very pleasant for my hands

Janko keyboards are currently in production branded as Chromatone - https://chromatone.jp/