What would be interesting would be if some language offered the ability to do thread-local (or thread-heritable) dynamically scoped monkey patching, and closure

What would be interesting would be if some language offered the ability to do thread-local (or thread-heritable) dynamically scoped monkey patching, and closures over monkey patches. So you could make a function that rebinds the implementation of lists, and can return a list that is for example backed by a mmapped file.