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?
sslh [1] (which is packaged and included in may linux and BSD distributions) has grown to support HTTP, SSL, SSH, OpenVPN, tinc, XMPP from the same port. It claims it can be extended to filter anything that can match with a regex.
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).