Emacs can already embed a webkit widget, but much more work would be needed to make interaction with the widget "emacsy". At the moment all you can do with it is send JavaScript snippets to it to make it change internal state. It would be great to intertwine this more with Emacs so that e.g. completing-read could be used on the DOM.
It would be rather fun to download a webpage in "BrowserEmacs" and have a REPL available with the page turned into a S Expression for you to manipulate at your or your plug-ins leisure. Throw in the tools to make plug-ins have there own UI and you have a developer's browser.
Ubiquity https://en.wikipedia.org/wiki/Ubiquity_(Firefox) (now defunct) had natural-language commands with localization including subject/verb/object order. It supported subscribing to commands too, so you could keep up with changing APIs automatically. It even understood pronouns, so you could write "email this to Dave" and it would email your selected text to your gmail contact named Dave. The only thing it was missing was a permissions model to limit what untrusted command code could do.
