> I think it boils down to having specialized editors which you embed, and to a common interface that you may define over that.

Emacs modes, in other words?

Like how, in Emacs, C-n nearly always does "move to next line" but it could mean "highlight next mail message" and "Enter" nearly always does something with the current line but it could mean "open currently highlighted message" or "newline and indent as per language-specific rules" or "send this line to a subprocess" or whatever.