I think markdown is probably as fancy as it can get and still be readable in the terminal after I clone the repo, or any ide, or the browser.
I think of it as a more formatted man page. Would I want a man page to be interactive? Or only work in a browser.
I also think if GitHub allowed more flexible markdown (like allowing html and js) it would turn more into a MySpace chaos. I don’t think authors would limit themselves to nice functions only once you let the JavaScript and CSS cat out of the bag.
I think your example is neat but that’s what pages is for. Use pages and add a link at the top of your readme. Just because GitHub is the first thing your users see doesn’t mean it has to be. Make a project web site, direct users there. Use GitHub pages for that if you don’t have a site elsewhere.