Marble, slate, granite. Iron, copper, silver, gold.

Just wondering if they are planned to be added to the terrain editor later on.

I don't know at this point. Nothing planned.
I'd love to have some way to spawn resources and ores but it's probably impossible to have a somewhat working UI for that on a 2D grid.

I think designating hotspots would be useful, right now im designing my second map and im trying to get an even distribution of iron across the map. one of the main issues i had on my original map (Seraphim Isle) was that the central region had a palteau area + a big mountain, so the center of the map took the majority of the iron nodes while the rest of the map and mountains ended up being very sparse. With my new map ive taken this into account but regions where i have hills that i want to have iron end up not having iron unless i tweak the terrrain to a point that there may as well be a mountain in a non-mountainous region of the map.

so being able to designate areas that are iron 'hotspots' (Perhaps like the quality hotspots?) and regions where you dont want iron (lets say on my original seraphim map, under the plateau area) so you can control the distribution of ores.

it can work on a 2D grid as you can assign areas where iron should show up or not show up