I think having a system that determines the 7 elements based on a meta organization of the elements would be a good idea.
using your division of the elements at
http://elementscommunity.org/forum/index.php/topic,4126.100.htmlSpindle 1 - Material Energies - Earth (Solid), Fire (Diffusion the direct connection between solid to gas), Air(Gas), Water (liquid)
Spindle 2 - Transversal Enegies - Life (Coherent Energy), Gravity (Internal Energy), Death (Incoherent Energy), Entropy (External Energy)
Spindle 3 - Logical Energies - Time (defined), Light (positive), Aether (infinite), Darkness (negative).
we could have 4,5,or 9 fairly easily.
4) same spindle
5) lock the spindles orthogonal to each other and include the main element plus the 4 closest other elemnts
example 1:
example 2:
9) main element + other spindles
You have a good point about the value of primes. I think that the 5 option would have the best of both.