I've been thinking of writing a text editor (ncurses). It will use scheme so you won't have to fuck about with its own custom language.
Notably it will be multi-modal and not require key "chords" or force usage of ctrl- or meta-. For instance, you could bind mode switching to # or some other lesser-used key, type #g and suddenly you're in "greek mode". Obviously you aren't limited to the default modes and any key can be bound to anything. You could also use it as a general-purpose tr and pipe input to it: echo "#glambda" | this-text-editor
λαμβδα
Also a possibility are macros, where you type whatever you want in, hit space twice (or sonething, or even nothing at all) and it converts accordingly, e.g. "lambda " -> "λ"
Other things include that it will auto-indent and all that standard stuff, but you won't have to delete all the preceding whitespace if you accidentally type a newline four tabstops in.
I'm also toying with it "learning" and understanding lexical scope for autocompletion.
The general idea is that it should be let you write the programs without the language's syntax or the editor itself getting in the way (once you learn it, of course).