I don't create original layouts. Instead, I modify existing layouts to incorporate a dynamically defined key that I call the "magic" key. To date, I have only put any real thought into the Magic Sturdy layout, and all others should be considered rough concepts at best. As a general rule, I design layouts for a columnar keyboard with a modest stagger. I will some times point out different options for row-stagger users, but I'm not looking at the problem from that perspective, so I'm certainly missing optimizations.
The magic key is a key that dynamically adjusts its output based on the most recent previous keycode.
The implementation that I use is the Alternate Repeat Key in QMK that is part of the Repeat Key feature designed by GitHub user @getreuer.
The output of a keypress of the magic key is determined by a function that takes the previously tapped keycode along with any modifiers (shift, ctrl, alt, etc) and returns a keycode (plus modifiers). The returned keycode can be a simple keycode corresponding to a single character or something more complicated, like a macro that corresponds to a series of multiple keypresses.
For example, in Magic Sturdy, the magic key outputs an L if the previous key was R, and it outputs Y if
the previous key was one of C, P, D, G, and W. Additionally, in the deluxe form, it outputs THE, if the previous key was space, tab, enter, or ". There are many more mappings, but this provides a
taste of the power.
I also use a repeat key, which, like the magic key, dynamically adjusts its output based on the most recent previous keycode. This key generally simply repeats the previous keycode, such that L + repeat outputs LL.
On a few occasions, I override this default behavior to have the repeat key act as a second magic key when
pressed after keys that are rarely repeated such as I and A.