The 3-D printing industry has been moving toward 3MF as a standard file format. It’s an XML-based format that claims to offer extensibility, interoperability, and freedom from the problems of other formats. The specification includes an XSD schema. I’m no judge of how suitable it is for 3D modeling, but yes, it is extensible. In fact, it’s designed with a relatively lean core model, so additional features can be added as extensions.
A recent Fortune article, “Why These Big Companies Want a New 3D File Format”, discusses 3MF from a business standpoint.
The old STL format, based on tessellation, is widely used, but it’s been criticized for generating huge files and lacking features.