Zen mode: Everything is hidden except the editor.
Typewriter mode: where the scrolling is can be fixed to one part of the screen and the text that’s being currently edited can be highlighted.
Both of these features are available in MD editors like Ulysses and iAWriter.
This would be nice. Also an option to toggle full screen on the active pane that would remember where multi pane setups were previously and return to that state when fullscreen was toggled off.