First I want to express how amazed I am by this project and its results ! It is simple, efficient and highly credible.

Looking at the OBJ file export, I was curious whether it would be possible to get a list of materials in order to be able to texture the buildings in 3D softwares (e.g. Blender).
Indeed it seems in the city visualizer that the roofs, walls and important buildings are already identified and can be coloured independently. So they could be assigned a material in a MTL file accompanying the OBJ file.

Is it something already planned or could it be included?

Yes, it is possible to assign materials to meshes in OBJ and export them as MTL. Probably I should do it because the same functionality is already implemented in Procgen Mansion. The problem is that I don't use (and export) UV coordinates because all my materials are solid colors, and for this reason applying textures is problematic.

That would be awesome. And, if I'm not mistaking, actually UV coordinates would not be an issue as long as geometry nodes are identified with  a single material. It is always possible to unwrap the faces in Blender afterwards and to rearrange textures along UV coordinates if needed. Being able to distinguish roofs from walls would already be a huge step.