The idea that really excites me is 'STRUCTURE editor as a language' - that is, the same sort of grammar as Vim, but manipulating semantically meaningful structures like expressions or functions instead of text. I'd love to see an editor reenvisioned completely around that concept, with "normal" text editing only as a fallback.

(Of course, Vim does have text objects, which are fantastic - one of the chief reasons I use Vim. But they have limitations; I think you can go a lot further.)