@Zach_Kaplan IMNSHO (e.g. “my opinion”), for archiving an easel project, the Easel developers get to decide on the file format, this is not an exchange (import/export) scenario, it’s “merely” backup and restore.
That said, you have to keep in mind that you will want to change the format at some point, so you have to be rigorous about being able to restore from really old project backups into newer versions.
Also, less proprietary is usually good, maybe a combination of formats, zipped up. So portions of the data that align well with an existing format, use that existing format, and tie the files together with a control file. As a totally made up example, say that .SVG or .STL was “the perfect” format for representing a single layer of an Easel project, but totally inadequate for representing all the project data and relating the layers to each other. In this case, perhaps a single .SVG or .STL file per layer and an XML file that references all the layers, along with project details that are specific to Easel.
-Kelly