When you design a map you put several fixed fields and ports, but also mark a lot of places as "suitable" to have a field. These places have "deactivated" and "invisible" fields that could be built via goons or M3. These places are shown in the map as "suitable" field places, when you drop enough goods over it the suitable place is replaced by a very small field with a single grass runaway, 4 acks and 2 FHs owned by your country. These fields have no city nor supply lines. When its acks and hangars are destroyed, the field is "deactivated" again. It dissapears and is replaced in the map by a "suitable" place again. This way we'll be able to have "temporary" fields much closer to the enemy fixed ones.
As an added feature, these fields could have only early plane-set available.