The CS department at the Ohio State University built their own language on top of C++ called RESOLVE/C++. A lot of students absolutely hated it, but it introduced a pretty awesome paradigm for working with objects: swapping.

If a function needs to operate on an object, rather than messing with pointers or copying the object, the function instead "swaps" the object with an initialized object. The function's "contract" specifies that it "consumes" the object that is passed in, so in code that calls that function, it is apparent that it better copy the object if it needs to access it again.

I think this provides a nice paradigm for allowing for immutability without excessive copying. The only functions that actually modify objects are the object methods themselves.

The RESOLVE/C++ language has been around for well over a decade, but it hasn't garnered much interest outside of academia. It's a shame, because it's a reasonably high level language that allows for mathematical proofs of correctness while still compiling down to machine code.