GLTF has extensions which allows for the definition of attributes/properties.
The EXT_mesh_features extension allows that geometries in a gltf can be given a specific ID.
https://github.com/CesiumGS/glTF/tree/3d-tiles-next/extensions/2.0/Vendor/EXT_mesh_features
Taking if further the ID can be linked to defined schemas with the EXT_structureal_metadata extension.
The structural metadata extension
https://github.com/CesiumGS/glTF/tree/3d-tiles-next/extensions/2.0/Vendor/EXT_structural_metadata
Both of these are used by the new 3DTiles 1.1 spec - adding these extensions would be required for 3DTiles 1.1 support. In the case the 3DTiles 1.1 support does get added it only makes sense the the GLTF writer be improved to also support this.
