This chatter stems from the first look at the season, which features an ailing Scully telling Mulder “You have to find him and you have to stop him before he releases hell on Earth.”
Given the various directions The X-Files goes with storylines, it’s kind of hard to predict plots based off of that statement, but some are thinking William might be the aggressor in Season 11.
There could be a ton of people who fall under the category “him,” but with Mulder alive and well it appears Scully did make contact with their son, and things may not have gone well considering her condition in the teases we’ve seen.