I suppose what I was picturing first was something akin to Conquest of the new world, where "gold vein" was an object on the map at the centre of a circular mask of boosted gold/mineral production. The same with fertile land. You had a decreasing bonus with distance. So in reality if placing a single farm you'd place it directly on top of it. Which would be quite similar to elemental now, with a mask of 1x1. In conquest outside the mask/region was normal production, elemental would be 0 or minimum.
The difference to conquest, a game I did like, would be using terrain to generate interesting shaped masks, between two mountains or along a coast/river. To be able to change them, and to have different ways considered of controlling them. (other than being the first to spot them). Even with no blank masks, having the ability to change the mask by overland spells as part of a victory condition would be interesting. (especially because without blank masks and if the new mask had no bonus, you'd win by destroying your resources)
Really I would like your House Highgarden vs Morgoth battle and I think it'd look beautiful if done well.
Perhaps a new suggestion.... Have the size of the mask be adjustable by events, magic or research. Have fertile land masks grow over time in your borders, while mineral masks shrink or vice versa for a Morgoth type race? it would be a way of limiting resources in the late game if the mask shrunk down reducing your production. And require investment, magical or mundane to maintain production. (such as gaining new fresh resources from the enemy by trade or conquest)
But about mask/region control, I would like your resources to be safe from magical modification within your borders.
Key: //\\ mask edges, ---distance, x centre of resources, [town] city owner
//[You]---X--)--(--\\-[enemy]
if the borders ( and ) shift:
//[You]--)(-X------\\-[enemy]
you'd still be within the mask of the resource, but the enemy would control it, and therefore be able to corrupt it. That would disasterous if it were your main food supply and they simply sealed the resource, or changed its mask area, or turned it into swampland.
Still, for it to be fun I'd like some resources to be different, have more potential. Eg tier 1, 2 or 3 resources. Split into groups in whatever way, groups of living resources, material, magical, unique... or simply rarity. You can use magic to manipulate within a tier/group.
A victory condition of X tier 3 "dragon graveyards" would require to find and control X powerful tier 3 objects, say an adamantium vein, life spring or whatever fitted into that category. These would be hidden at the start and be interesting when fighting against someone who had Y tier 2 objects or Z tier 1 objects. Picture 3 races fighting all looking at a simple iron resource as either good for troops weapons, good to prevent someone turning it into an alchemists or one small part of the X alchemists victory condition.
Perhaps, rather than a victory condition, they could instead simply unlock an 'ultimate' unit or spell, dragon graveyards would generate a bone or ghost dragon, fertile lands could unlock a 'life force' overland enchantment type spell making all your units regenerate.