I would really like to see some OOP language that implements James Reed's reference versioning or at lest a weaker implementation where every object "mutation" creates an effective copy with the changes, without touching anything that would be referenced previously. It would fix a lot of low-level annoyances people have with SmallTalk, JavaScript and so on. Would be extra amazing if it was based on prototypes, not classes. But at this point it sounds like I would have to write it myself.
Those structures don’t behave as objects themselves, but it’s not uncommon for Haskell monads, for example, to have very object-like behavior when you build them up a high layer, while each step in the monad is still an immutable value with cheap mutation-copies.
Actually, class-based handling of raw data can be done very nicely if you have multiple dispatch, which solves the so-called expression problem. Cf Julia.