Automatic ssh multiplexing

I want ssh to automatically multiplex connections. When I open a second connection to a host, it should automatically be done over the first TCP session. I want easy persistence configurability. Moreover, I want to be able to type "@remotehost echo 123" to execute any command on a remote host. Persistent connections should survive reboots. By default the need for persistence should be determined by how much I frequent some host.

I want easy forwarding to handle situations where I can only have one outgoing TCP session. It looks like another topic, but if the solution is centered around OpenSSH, it goes here all the same. I want to be able to forward X11 through several SSH hops.