For various purposes it's useful to pass some more or less fixed string from the client to the server side. E.g. when forwarding sockets, the remote socket name needs to include some string unique to the local side (like the host name) to avoid collisions between connections from different hosts. Think gpg-agent forwarding or using emacsclient on the remote host to open the file (via TRAMP) on the local host.

This is currently possibly by using a wrapper script around ssh that sets some environment variable, but setting an environment variable directly in ssh_config would make things a lot smoother (no messing with PATH required). Being able to expand environment variables and use expansions like in the Match directive would be nice, but even a fixed string would go a long way.

E.g.:
=== Begin .ssh/config ===
Host *.localdomain
   SetEnv EMACS_SOCKET=/home/sascha/.emacs.d/server/server.%l
   SetEnv GPG_AGENT_INFO=/home/sascha/.gnupg/S.gpg-agent.%l:1:1
   
Host myserver.localdomain
   RemoteForward /home/sascha/.emacs.d/server/server.mydesktop:/tmp/emacs8193/server
   RemoteForward /home/sascha/.gnupg/S.gpg-agent.mydesktop:/home/sascha/.gnupg/S.gpg-agent
=== End .ssh/config ===

(As you can see, environment variables and Match-like expansions would be useful in RemoteForward as well, but that's a feature request for another day).