I had this discussion with an autistic male who happens to be in his 30s. All I said was that the game should have killable children, more nudity and references to love and rape.
Fallout 4 literally is the least mature M rated title I've seen. There is hardly any limits being pushed.
Adding those things just to "push the limits" of the M rating sounds like trying to be edgy just for the sake of being edgy. At the same time, though, banning child-killing strikes me as the work of a total weenie. You don't need to have a major plot-point be about a guy who throws children into meat-grinders for fun, hell, you can even have the other NPCs never fight them at all. Just don't make random characters arbitrarily invincible just because they're smaller and more annoying than most others. There's no need to appease whiny starfishs who can't properly differentiate a smear of red pixels on someone else's screen from something that actually matters. Let players play the game however they want, and everyone goes home happy.
Unless of course they have some kind of mental defect that causes them to get irrationally angry at the way a different person plays their copy of a singleplayer game. The same kind of people who start crying when they hear about Dark Souls getting an easy mode or whatever. Those kinds of people can feel free to join the rest of the children in the meat-grinder.