Software compatibility in general. There’s still a lot of Windows-only software out there that people rely on.
Also, security-by-default for apps would be nice. Snap and Flatpak are great starts but it’s still to difficult to manage and too easy to install non-sandboxed software. Some random weather app should never have access to your photos, camera, file system, networking, etc… without the user explicitly granting permission.
There's Qubes, but even its enthusiasts are quick to say in its current incarnation it's not in any shape to be foisted on an end user. The other Linux flavor that sandboxes apps by default would be Android, which seems to have a few users.
Also, security-by-default for apps would be nice. Snap and Flatpak are great starts but it’s still to difficult to manage and too easy to install non-sandboxed software. Some random weather app should never have access to your photos, camera, file system, networking, etc… without the user explicitly granting permission.