Is this along the lines of what Editus does? Or different? If the latter, what would be the main differences (if known at this stage)?

I have to admit, after using Medium for a few years, I always feel the WordPress editing option is a poor second, and could do so much better (especially given the userbase).

11 years of WordPress use, and still essentially the same clunky WYSIWYG interface. It'd be nice to see what you're creating on the fly.