Minecraft Ore Distribution is Very Unrealistic. You cannot find diamonds next to iron in real life. You would have to find an unique area to mine iron or diamonds. This is why people moved to California to mine gold. But my idea would change it.
-Ores would be found based on area instead of depth.
-Ore deposits might be more likely to spawn in specific biomes (For example, Coal deposits would be found more in mountains then anywhere else)
I would say this mod might be fitted more for Multiplayer because of how much effort it might take to find ores. This could encourage Players to colonize areas to get access to unique ores like tin or copper. it would also encourage trade with other nations to attain a resource a nation doesn't have in their territory.