Some ideas: it would be nice if you could run it inside a web browser. You could print data-structures, and like in the developer console, you could navigate through data-structures by unfolding them. You could also do console.log(image), and the image would appear inside the browser. Or you could type "admin()", and a menu with buttons would appear below the command prompt. Etcetera...
Definitely good ideas, but there's some tools that do similar things, and I wouldn't want to tread on the other libraries' boots. I forget the name, but one popular library creates a web server where you can open chrome web tools and do profiling on the fly, which is awesome.
Vantage.js is entirely based on TTY controls, so it wouldn't really work too well in a browser.
(Just thinking out loud).