The only proposed rule that makes sense to me is to make the share price a floating value. Such a fund isn't cash if it's invested in securities that have some kind of maturity, even an overnight maturity. Money market funds belong in the general category of actively managed fixed income funds but brokerages are reluctant to break this bad news to clients. It will mean one less cash management tool in a toolbox already bereft of yield thanks to the Fed's zero interest rate policy. Wealth management firms had better quickly find some other place to sweep overnight balances if they want to avoid an MF Global kind of collapse in the next surprise credit crunch.
I'm more than willing to use Bill Mahr's method here. "New rule: A money market fund is no such thing if it doesn't hold money (as cash) and can't be exchanged on a market (due to illiquidity)." That wording should be easy enough for the SEC to implement.
Full disclosure: No position in any money market funds at this time.