Possibly, depending on how much demand we see. You can obviously kind of hackily use coffeescript right now by just requiring the package, but that's not a real solution.
Additionally... Tonic is comparible to CodePen in terms of testing and thus gave me the idea;
Perhaps you could also include other languages into your platform. Such as a Stylus REPL, SASS, SCSS etc. all of which, like CoffeeScript, are just JS modules on npm.
With compilers like Stylus and SCSS you would probably default the return value to be the compiled CSS, and with Coffee you'd need a toggle button to show what the compiled JS looks like, though not 100% necessary.
With that said, if there was just a way to set up a "build" script for a REPL to set up the environment to achieve the same result. As in, you require('stylus') then have some global variable stream/buffer, tonic.output, for which you could inject into the module of your choice to generate output, and then save it via tonic.buffer.write() or something. It could then be ridiculously flexible in terms of testing out finicky new languages/compilers.
Any plans to support compile-to-js languages? Coffeescript, etc.