The Xbox Series X will come with 1 TB of SSD storage as standard, and will be compatible with an Expansion Module designed by Seagate. The latter has now been unveiled and will run on two lanes of PCIe 4.0.
https://www.notebookcheck.net/Seagate-reveals-new-details-about-the-SSD-module-for-the-Xbox-Series-X.482669.0.html
I hope it will be possible for someone to create an M.2 enclosure for this.