This isn't cast in stone yet, but I'm told that starting Monday, a pilot program will be filed requiring all exchanges to apply new rules to S&P 500 stocks and select active ETFs, designed to prevent a rehash of Thursday May 6th's extreme price action.
Trading in each stock will be monitored by the exchange it's listed on. Curbs will be triggered when there is a 10% decline (or advance) in any rolling 5 minute period between 9:45am and 3:30pm. If the curb is triggered, the listing exchange will send out a message, and trading will be "paused" for 5 minutes, during which time no trading would occur - quoting only. Once the listing exchange prints a new trade, the pause would be lifted. This is similar to the way that stocks come out of breaking news trading halts currently.
The plan will likely be implemented in 30 days.
EDIT: in case it's not clear, it seems that the goals of this plan are to 1) allow for "circuit breakers" during times of large price movement and 2) to put the onus of responsibility back on the listing exchange - under these rules, the listing exchange is the one who controls the "re-opening" of a stock.