Multiple resources on one plot would be a lot more work, as we'd have to change the underpinnings in the DLL, and therefore much more error-prone I think.
However, suppose we had hidden proto-resources (map resources which are present but nothing reveals them). Then we could have the DLL (saves changing the map scripts, but it could be the map scripts if we had to go that way) place proto-resources on many of the plots that initially lack directly map-spawned resources according to some specific rules (see below). The prospecting actions would then essentially convert (or have a chance to do so) a proto-resource to a regular map resource.
Suggested rules:
- Each proto-resource represents a group of real resources the act of prospecting can possibly turn it into (e.g. - proto metals can become any of the regular metals say)
- Each regular map resource specifies which proto-resources are likely nearby (with probabilities). This information is used to initially place the proto-resources on tiles adjacent to map-script-placed map resources according to these probabilities.
- Workers get promotions that allow prospect actions, with the promotions unlocked by techs
This (example) if a map script places iron, then the DLL is likely to fill in proto-metal hidden resources nearby, which can later be prospected into actual meta resources in the same group. That way the map script still controls the type of resource placement, and (depending on how tightly we draw the groups) it retains control over the overall distribution.
Techs now have new value in unlocking the ability to prospect, and workers gain useful promotions. Indeed even entire promotion lines such as:
PROMOTION_METAL_PROSPECTOR1 - can prospect for copper, lead and iron
PROMOTION_METAL_PROSPECTOR2 - can prospect for aluminum
PROMOTION_METAL_PROSPECTOR3 - can prospect for uranium
Alternatively (perhaps better, so the player doesn't have TOO much control):
PROMOTION_METAL_PROSPECTOR1 - can prospect for metals, and may discover any of copper, lead and iron
PROMOTION_METAL_PROSPECTOR2 - can prospect for metals, and may now also discover aluminum
PROMOTION_METAL_PROSPECTOR3 - can prospect for metals, and may now also discover uranium
i.e. - you can' choose what you're looking for beyond the broad group (metals in this case), and what you find depends on probabilities defines on the proto-resource, with only those that the promotion enables considered.