Not quite, I'm talking about replacing the various important java interfaces in clojure.lang (IFn, ISeq, IPersistentMap etc) with protocols, so you can implemen

Not quite, I'm talking about replacing the various important java interfaces in clojure.lang (IFn, ISeq, IPersistentMap etc) with protocols, so you can implement them yourself without using any Java-specific language features. Dunno how feasible this is, but if a language is going to define its own abstraction features (here, protocols) on top of the underlying platform, it would seem cleaner to eat ones own dogfood in using these abstraction facilities for the core abstractions which the language and its core library are based on.