Windows Central has an overview of the latest rumors it has heard about the upcoming user interface changes and the new features of the 21H2 update for Windows 10. Expected to be released in Fall 2021, this update will feature the "Sun Valley" reinvigoration of Windows 10. You can read a full list and view some mockups over here.
One of Microsoft's goals with Sun Valley is to freshen up the Windows 10 user interface a bit. This will include rounding off corners on buttons, app windows, and shell elements such as the Start menu and Action Center, making legacy interfaces like File Explorer, File Properties, and copy dialogs more consistent with modern interfaces in Windows, and cleaning up parts of the OS to create a simpler, leaner user experience that caters to today's more modern workflows.