Unintended Acceleration, Unintended Consequences
The changes for brake override, where engine power is reduced when the brakes and throttle are pressed simultaneously, have mostly to do with software and should be fairly straightforward to implement.
However, one unintended consequence could be that it will no longer be possible to heel-and-toe a car equipped with a manual transmission. That's a technique used during performance driving, where the driver "blips" the throttle while shifting and braking to keep the engine at a higher RPM for more and faster power delivery once the next gear is selected.
Event Data Recorders (EDR), also known as "black boxes" will be a bit more involving. The costs will vary greatly with the rule's requirements. I worked on this issue for 6 years for heavy trucks and there are a lot of things to consider. They've also been required to record data a certain way on passenger cars IF those cars were equipped with such a device. Typically, manufacturers install them to trigger and record data to protect themselves from liability.
NHTSA's cost estimate for these devices in the 2004 rulemaking was laughable, the low end starting about $0.50 to install them on each new car. Let's hope whatever comes out is reasonable and effective.