I like the idea of immutable-by-default, and in my own musings on this I've imagined a similar thing except that instead of a mutable keyword you'd have something more akin to Python's with blocks, something like:

    # Immutable by default
    x = 2
    items = [1,2,3]

    with mutable(x, items):
        x = 3
        items.append(4)

    # And now back to being immutable, these would error
    x = 5
    items.append(6)  

I have put almost zero thought into the practicality of this for implementation, the ergonomics for developers, and whether it would be different enough and useful enough to be worth having.