I wonder how far you could take this in terms of protocols. I can't think of a good use case yet, but is it at least technically possible to detect and quickly proxy away most common protocols?
What I find lacking lately is I've mostly wanted to extend the forwarding/routing of ssh connections based on username (or better by identity) to different VMs or hosts, but I have no idea how to achieve that at the moment (without creating dummy users on the sshd server).