While slasher movies existed before Halloween, none had broken into the mainstream (beyond, I guess, Psycho) and without Halloween, it's almost certain you don't have Friday the 13th - at least, in its most known incarnation. That movie was pretty much green-lit after Halloween's success - not Black Christmas' or Texas Chainsaw Massacre's success. That isn't to say those films didn't influence Friday (which, like Black Christmas, had an unknown killer), it's just that those movies were already six-years old when Friday was made and, for the most part, not widely known (though, TCM saw a bigger stretch of success).
I mention Friday because, beyond Halloween, it set off the slasher craze of the 1980s. Without Friday, and the rash of sequels and knockoffs, slashes probably sputter into the 80s and likely do not become the prominent sub-genre of the horror film industry - at least, without another major slasher movie filling the hole left by Halloween.
It was too late for Black Christmas and Texas Chainsaw Massacre. Paramount was not going to hop on board the Friday train on the idea that two movies, six years prior, had done okay enough - it was fully a direct result of Halloween's success (and Halloween grossed $40 million more than TCM - a margin that actually was greater than TCM's entire run ($30 million).
So, yeah, horror in the 80s is probably different. Paranormal movies potentially become the bigger craze (Poltergeist was a template for success) and without the slasher mania of the 80s, it doesn't bleed over into its revival in the 90s with SCREAM - so, no clones of that movie, either.
Then again, maybe another movie comes along and shakes things up enough to keep the sub-genre alive. It's possible Wes Craven still does A Nightmare on Elm Street, but it's a bit different without the success of Halloween & Friday the 13th - with that movie really becoming the template to the new style of slashers. If that's the case, the genre probably balloons throughout the late 80s (instead of dying off toward the end of the 80s before its revival again in mid-90s) and carries over into the early 90s. From there, who knows what path it takes ... but that's assuming Nightmare even gets made in this universe.