It would be cool to be able to change the various parts of the terrain that are usually generated by the seed. This would include the height map, the various rocks that prevent building/road placement, ore generation, and (presumably once they’re added) biomes and their various features.
One of the cool things this would allow is the creation of art pieces. I can already imagine people using the various buildings, roads, and ores to create what is essentially both a functioning city and cool pixel art at the same time. Or maybe people re-creating the layout of real-life cities.
Something else it would allow is greater ease in testing complex city layouts. The ability to create a perfectly flat world with custom ore placements would allow players greater freedom in messing with the traffic coming from the various mines, as well as the efficiency boost that proximity to the factories that produce a given building’s consumed resources.
I understand that depending on how the terrain is generated in the first place, this may be quite an unrealistic task. A more pared down option might be to generate a perfectly flat world where we can only change ore placements, or maybe just the ability to place mines anywhere on the map, regardless of ore, and still have them produce (essentially making them function the same way oxygen, carbon, and water already function).
If this is too much work for the devs, then I guess this is an idea for modders to use once mod support has been implemented, similar to how WorldEdit and TerraMap work for Minecraft and Terraria, respectively.