FME Server projects are already a great way to migrate workspaces and related resources between FME Server instances in a DevOps-style way, without having to manually re-publish through FME Desktop. This would be even more powerful with the introduction of deployment slots similar to how they work in Azure AppService, as it would allow updates to existing processes to be deployed and tested in the target site before replacing the live deployment. This would be particularly useful in environments where there is no pre-PROD instance available.
It could work at the project level and essentially create a parallel namespace that resources get deployed to, and where they can have the same name as the live ones, but with different settings. As an example, a web service or database connection called 'MyDbConn' would point to a pre-PROD database while the same database connection in the live deploymen slot would point to the PROD instance of the same DB.
Once everything has been tested and is working OK in the parallel namespace, the live slot and the parallel slot get (hot-)swapped to go live with all updates at once, ideally with no service interruption.
If any issues were found afterwards, the old configuration could be restored simply by swapping back, and then updating/testing the resources in the parallel namespace until they work OK.