CoffeeScript played an important role in the development of ES6, but looking forward, it seems to have a same-but-different set of features vs. actual JavaScript, which may doom its future.
Hell, I'm still waiting for Array::map to be dependable.
Ultimately though, wouldn't you want to use a js preprocessor that lets you write full spec ES6 and compiles down to ES4 or whatever your runtime target is? That's available today, and it doesn't confuse the meaning of class, fat arrow, variable scoping, etc.