Desktop as a service. With the latest feature being worked where apps can be handed off to another compositor, I want the next stage where my compositor and desktop can be swapped with my intervention or notice. Wanna do redundancy? Running the backup live as a hot swap. Wanna do live updates with no interruption? Start the next compositor, try and loads the apps, if nothing breaks, swap the user, if the user doesn't hit the notification to revert kill the last session.
Add in better remote compositor support and it can get really cool. Allowing for a distributed DE across your devices. Making high availability more possible as well, but that might actually be overkill.