I eagerly await the day when Toady allows us to mod GLOBAL mineral occurences.
As you are probably aware, currently we can mod occurence rates of the minerals in DF. However, these are usually overruled by global placement preferences, that define where which mineral will appear.
DF should do geology better than that. Each mineral should (in addition to the current environment tokens) have a biome affinity, layer affinity, and placement type, and get distributed throughout the world according to that. By "placement type" I mean stuff like coal being present in vast continuous layers, while kimberlite is usually found as vertical tubes. Layer affinity is the general "depth" on which a mineral is usually found, or rather a depth range at which its appearance is most probable - on the world scale, not local scale, though that may too be included. Biome affinity is more suitable for soils. For example, how many temperate broadleaf forests have you seen growing on red sand? That kind of thing.
All in all, the system will lead to slightly more interesting fortress-building. For example, embarking on a specific type of layer, you may or may not strike a multi-million-ton gold vein. This will make searching for a "perfect" spot a lot harder, but isn't challenge one of the main points in a game?