JSON file format for Type and Format files
The XML files can become quite difficult to work with especially with just a text editor. Most of the file is made up of tags and almost no actual functional data. JSON would be easier to work with and continue to march away from XML configuration files.
Derp McDerp commented
I'm all for deprecating .ps1xml files but JSON isn't the way to go.
PowerShell's already existing .psd1 files are superior to JSON.