We're expecting to hear about Microsoft's new version of the Xbox app for PC during their E3 conference, but thanks to @h0x0d on Twitter we can see the beta version of it sitting on the Microsoft Store right now. It will apparently be a way to "discover and download new games with Xbox Game Pass" as well as a social system for connecting with other players. It sounds like a total replacement for the Microsoft Store and, honestly, it's about time.
Features listed include the ability to "browse the library of Xbox Game Pass games on PC" sorted by genre, with reviews and suggestions for similar games alongside the expected info like trailers, screenshots, and system requirements. It'll have a sidebar for installed games, with the most recent ones at the top and a chat system that promises to work "across PC, Xbox One, and mobile". It'll also be compatible with the Xbox Game Bar.
Here's some screenshots so you can see it in action.