Recently an Xbox Insider Program update made an app called "Mercury" available to a few testers, with a splash screen that just reads as "Coming Soon. Please upgrade to access the latest and greatest."
I received a few messages about it, and thought I'd try and find out some more about what it is and what it does, since some had speculated that it might be something related to Project xCloud.
After doing some digging, it seems that this app is the codename of an upcoming Xbox Store redesign, rebuilt from the ground up on new architecture, designed for 4K TVs. This new store app is also destined for use on next-gen systems like the Xbox Series X, and long-rumored Xbox Lockhart, as well as current-gen Xbox One systems.
If you try to install Mercury from the Microsoft Store to your console right now, the download simply fails. But it probably won't be too long until testers in the Alpha and Alpha Skip Ahead rings are able to take a closer look at it.