In GOA, as with a lot of games, supply sources are generally only 1 hex - a city or a port or some otehr specific designated point.
Suply is normally unlimited.
However to be honest there is a limit to how much a single hex can really support - cities aer often short on fod - they have to import it from the countryside of course...that's what countrysides are for!!
But in GOA we have HQ units with activation points that represent massive stockpiles of everything needed for major attacks - ammo, equipment, food.
So i'd like to see supply function through HQ's, and also the size of the avaialble "supply hinterland" being taken into account.
Eg if a unit is cut off in a single hex, it should lose supply quickly as there isn't much infrastructure to support it. But it it is cut off in a "pocket" of a few hexes then it should lose supply at a lower rate....or even not at all perhaps if hte pocket includes a city and a few country hexes representing an almost complete infrastructure over a wide area.
HQ activations points should delay loss of effectiveness by some time - I'd suggest a whole turn per point.
Just some not-particularly-random thoughts.......