I have done some more doodling and pondering, and have decided that the Economic map should turn the Triangle design of the Calhamer map by 90° by faciliating the Northern Triangle (E, G, R) and the Southern Triangle (A, I, T) - both of which are usually secondary theatres to the main action in the Eastern (A, R, T) and Western (E, F, G) triangles.
The northern triangle would get a moderate amount of neutrals to divide among them, and the southern triangle a greater amount (as they are among the weaker powers on the Calhamer board, either at risk of early elimination (A), suffering from lack of options (T) or being outside of the decisive Eastern and Western triangles and often reduced to sitting duck kingmakers after securing Tunis (I)). - This might possibly unduly strengthen the Eastern triangle (as 2/3 of the Easterners are also in the Southern triangle) and weaken Russia too much, but it seems to me like a risk trying out. I might have to fine-tune a v2 if it warps the power dynamics too much, but only actual play can tell.
France would take the role of Italy, as an in-between between the two major triangles with only one guaranteed build (similar to Tunis for I on Calhamer). France is generally the strongest power with both the best victory and the best survival record, so I think they can live being a second stringer on the secondary map, especially as they still get to be powerful on the main map, and can translate this power into units transferred to the Economic map if they desire so. - On the other hand, Italy could use its stronger position on the Economic map to transfer units from there to the Calhamer board, and have a stronger hand in the outcomes there if it desires so.
I am undecided yet on if I should leave every power at 1 starting SC+unit, or if I should up it to 2 SCs+units.
Another interesting option could be to leave all powers at 1 starting SC on the economic map, but give them extra units in the first year for additional early movement options in the first year. (I have seen something similar done in a Diplomacy variant about the invasion of Germanic tribes into falling Rome a few years ago, but am unable to locate that variant.)