Plan 9 is a research os that was made to explore the rigorous implementation of a few key ideas. You're missing the point if your criticism of it is that it doesn't run your favorite database.
If you like the ideas that Plan 9 implements you should apply them to you favorite big and nasty, full-of-compromises, gets-real-work-done OS.
For example, a key idea in Plan 9 is that all resources, from files and networks, to a processes runtime environment, to compute clusters should be represented as an entry in a single hierarchical namespace that is accessed with file semantics(read(),write()).
This is a really cool approach. It makes interacting with resources really easy. You can use any tools that manipulate files. It allows easy remote sharing and manipulation.
Want to use a printer on a different machine? Mount the printer and go. Want to write to a remote screen? Mount the framebuffer and go. Want to administer a running daemon? Mount the directory that represents it and starting messing around.
You can also create your own resources in this namespace. An example was a computing cluster with a file interface. Want to run a program on the cluster? Just mount the file an write your binary to it.
It is a neat approach and I would recommend you check it out.