They'd basically have to expand the roster if they did something like that. Probably to something like 65-72 players, which is insane. If they went with 18 games, they'd also have to adopt 2 bye weeks.
Having 2 bye weeks would solve the problem anyway...and honestly, I think that's the best solution. I get that the purpose of expanding the NFL schedule to 18 weeks is to make more money, and if teams had 2 bye weeks, they would just be playing the same number of games as they are now. But it would still allow the NFL to make more money because you'd have an extra week of TV coverage, an extra week of rivalry games, etc.
Yeah, I guess teams wouldn't reap the benefit of more ticket sales, but if you're having to consider new rules disallowing players from being able to play anyway, I don't really see how much you're accomplishing. Injuries are at an all-time high as it is. That extra bye week would also serve as an extra week of recovery. To me, that's a MUCH better option than nerfing defenses even more with new rules on how they can play the game.
I've even seen some arguments for making the pads smaller and not making the helmets out of such hard material as a way to REDUCE injuries because defensive players would be more conscious of how they're going in for a tackle, rather than treating themselves like a human torpedo. The comparison that's always brought up is rugby and how they don't have the same injuries you see in the NFL. It's an interesting discussion, but I think adding a 2nd bye week to an 18 week schedule would do a lot to help on all fronts.