Containment is becoming more prevalent in the computer world. We have containers coming to Windows Server 2016 (or earlier versions with WinDocks), a concept that's been embraced in the Linux world for some time. If you haven't looked at containers, they are a great way to isolate and stabilize your application environment. They can also be great for dev/test environments. You might do a little reading to understand how they can be used.
While containers might be good for SQL Server (or maybe not), we've had other types of containment in SQL Server. We got partially contained databases in SQL Server 2012, with little change since then. That is susprising to me since I expected that all Azure SQL Databases would be contained, and we would want more functionality inside of them. However, those have opened up slightly and I'm not sure where containment is going.
Personally I'd like to see jobs contained inside of a database. The vast majority of processes I've written over the years for application or system work on a database pertained only to that database. I'd like to see things like backups, maintenance, etc. contained as jobs inside a database, that travel with the database as I detach/attach it, backup/restore, and more. That might eliminate some of the issue with clusters and AGs if most jobs moved with a database.
That got me thinking. What else would we want to contain? The thing that comes to mind is a distributor. I'd like to have a distributor somehow contained with all the information about replication processes contained inside of this database (including the jobs). The database would need a way to register with the instance so the publisher and subscribers could find it, but wouldn't that make replication administration easier?
How about the SSIS Catalog and ETL information. Having more information here as a contained database might make managing ETL operations easier, especially if we need to move the load to another instance.
I'm sure there are other ways in which having information, metadata, and job functions contained inside of a database would be helpful, and easier on the administration of the system, whether by humans or automated systems.