These were examples of extensions to make a point and not at all what we'll be focusing on. What we're making sure of is that the platform for such things exists and enables all of the things that old UI's and such don't allow.
I think the parallel to emacs is a good one and while I certainly won't say at this point that we'll be the "modern emacs", it's our hope we can come close.
I think from the discussions and other threads what I infer is that we need a total re-look at the concept of an editor, Your way actually looks good. Except that we need to retain the 'Emacs infrastructure'.
We need to really keep Lisp as the extension language.
Lisp is the extension language :) Everything so far is Clojure + ClojureScript with the language backends written in the language they support (python in python and so on)
How about Stallman's idea for GUILE (Scheme) uber-alles. Do source-to-source translation of approximations of the supported languages to Clojure. This would allow everyone access to almost everything in the tool.
I think the popularity of CoffeeScript is a datapoint against your idea. I bet a lot of pythonistas use it because it's less of a shift than Javascript. Also, in such a system, if people are curious enough, they will learn the native implementation language as a result of the system being so constructed.
I'm pretty sure the core of it is to be written in Clojure, so yes, you'll need the JVM running. It is necessary if it is to be written in a JVM language. Eclipse and a number of other IDEs also require a JVM to be running.
I think the parallel to emacs is a good one and while I certainly won't say at this point that we'll be the "modern emacs", it's our hope we can come close.