the "it's actually modern day" twist

Han doesn't kill Williams; just beats him up and imprisons him. Bolo kills Roper then fights Lee and gets killed. Williams escapes with the rest of the prisoners and fights his way out while Lee kills Han. It would have been the perfect movie.

Opening massacre scene. I've always hated the full reveal of Pluto(who's 'officially' revealed in the trailer attack scene) and him swinging and bashing a guy on the pick ax a billion times. It looks stupid and is way overdone. My ideal version would be to cut out the majority of that sequence(showing only the first and second hits), so Pluto isn't revealed, and most of the violence is left off screen while you can still hear the violence, then it cuts to the one scientist whose running away, almost to safety, then is dragged down by the mutants. It'd be so much more effective this way!

The moms death in the trailer attack. I'd use the R rated version of it, which I find much more effective. The R rated cuts to a wall being splattered with a ton of blood and brains after a loud bang. The unrated meanwhile shows an cheesy looking up close shot of the mom getting shot in the head with minimal blood, then cuts to a few random shots of a tiny bit of blood on another wall, then the shot from the R version is used, but only a later portion when the grue is already on the wall and you don't see it splattering.

I'd also cut the brief point of view shot of the mutant holding the gun to the babies head. It's too much 'look at this cool shot' and takes away the impact and is best left cut out, considering you already see the baby with gun pointed at it in the same shot a moment earlier, so this 'cool' shot is redundant.

The jump scare stinger reveal ending showcasing the mutants still exist out there and are going to massacre the surviving members of the family. It's really played out and stupid and ruins the movie. The family has been through hell, and the way the film is shot and edited before that binoculars reveal is effective, emotional, well scored. Then it goes for a cheap cop out final 'scare' ending to leave it open for a sequel or something. The survivors need a 'happy' ending, they deserve it after all they've been put through. And while they may still die after the credits roll, since they're stuck in the middle of the desert with no working automobiles and no signs of life for hundreds of miles. At least leave the aftermath of their reunion up in the air instead of the cheap cop out they use.

And finally, remove the heavy metal song that plays over the last half of the end credits. It really kills the mood after the first song which fits the movie perfectly, then goes into a trying to sell a soundtrack mode by using some crappy new metal bands song. Either add a atmospheric score after the first song, or leave it silent, and just moody since the movies a roller coaster ride as it is.

Pluto's death, I'd remove the low angle shot from below of him being impaled with the pick ax,and brain/blood goes flying. While a 'cool' shot, it takes away the impact of the scene, since it's far more effective, cutting to the hero after seeing the ax smack his head. The pacing is thrown off with this brief 'cool' shot.