If we want to have more officials dedicated, say, to protecting the quarterback, why would the official have to be on the field? That seems very old-timey. In an age where we have a camera on cables zooming over the players heads prior to the snap, it would seem like we could have dedicated, high def cameras trained to solely focus on a QB, or perhaps on receivers downfield (to check for interference), or whatever. Heck, probably an individual camera for all 22 players on the field.
And it would have to be a “review”. It can be an in-game official, in the booth, staring at a screen, indicating to throw a flag live, and he or she gets to make that call without consultation with the other refs. When a holding penalty is called by one official, the other 6 don’t get around to argue whether or not it was real and look at a replay. That’s their job, it’s their call, done.