Please note that what I personally would like to have goes further (see my last comment), it is about the interchangeability of the for-in loop with forEach without changing the expression, so the programmer can concentrate on which kind of loop she would like to have (a closure at the end of a chain, or the ability to break from the iteration). This is a kind of user-centric approach, but I think a valid one. I suppose for that you need to adjust the compilation of for-in loops, adding an operator will not suffice.
(…And just as a cross link for the mentioned problem of for-in loops “loosing” type info: see this comment in another topic.)