Do you have a specific gripe against X11? Again, it would be very much against the Unix philosophy to roll that highly complex functionality into the core SSH protocol. The problem would still be just as complex and challenging. Implementing a network-transparent GUI always is. You'd lose the separation of concerns, and you'd end up running two GUI systems rather than one.
If you want a very basic GUI over SSH without a full-blown GUI like with X11, you already have the option of using a TUI like Midnight Commander. You can preview images on the command-line, with a tool like imcat. [0]
