Map WSL filesystems to a drive letter, for the sake of programs that don’t like the \\wsl$\… UNC paths
Since version 1903, there’s been the \\wsl$\foo construct for accessing WSL filesystems from Windows; but not all programs support UNC paths: cmd.exe, for example, does not support UNC paths as the working directory, so if you start cmd.exe from bash when PWD is on the WSL side rather than in something like /mnt/c, it’ll drop you in %WINDIR% instead.
There is a way of mapping a UNC path to a drive letter: subst. For example, `subst U: \\wsl$\Ubuntu`, and now you can access `\\wsl$\Ubuntu\home\me` as `U:\home\me`. However, when you start Windows programs from the Linux side, it will still use the UNC path rather than the mapped drive letter.
The idea is this: devise some optional way of registering a drive letter for the filesystems of a WSL machine, and then whenever it is generating paths for Windows, have it use that drive letter rather than the UNC path. The working directory when starting a Windows executable is the only case I know of, but there may be others.