Oops, I should have said basic "planetary energies," since I equated the nature of the planets (including the "modern" planets) to the positions, rows and columns in the spread, which isn't a comprehensive "astrological" spread by any stretch.
Regarding the "rows," Moon, Mars and Jupiter relate to the "aspirational" context; Venus, Sun/Mercury and Saturn to the more "deliberative" angle (a formative or actualizing phase); Neptune, Uranus and Pluto to the "transformative" urge.
In the "columns," Moon, Venus and Neptune relate to "motivational" factors, primarily rooted in the past; Mars, Sun/Mercury and Uranus speak to opportunities in the present; and Jupiter, Saturn and Pluto define future possibilities and related constraints.
For the cross-cutting diagonals, Moon, Sun/Mercury and Pluto blend historical knowledge with present awareness and future resolve, while Neptune, Sun/Mercury and Jupiter show how the querent can filter unresolved past issues through present awareness and bring external resources to bear on closure.
The central position of the spread, where Sun and Mercury sit in a kind of "dispatching" capacity, is envisioned as a "cross-roads" or "waystation" for the transitional energies at work in the spread, linking them according to a sort of "skewed" affinity: Moon and Pluto are the fastest and slowest energies, respectively; Venus and Saturn share a connection with both Earth and Air; Jupiter and Neptune both express different facets of the universal, transcendent nature of Pisces, and Mars and Uranus are similarly prompt and emphatic in their action.
This was a first cut; it contains the most well-reasoned distribution of the planets I could devise, and it necessitated a few compromises. I hadn't seriously considered bringing elemental and rulership considerations into it since, for simplicity of interpretation, it seemed like planetary energy and field of operation (similar to the "astrological house" idea) was enough to start. I will most likely find them sufficient in the final analysis as well.