I don't understand that "things are only good when only a few people knew about it". That is entirely subjected to bias and is rarely true, for instance, Slenderman was just a story on Something Awful, not even a very a good one, just a generic, scary character that doesn't even have any really unique characteristics. Furthermore, the appeal of Slender (game) is as basic as any other horror game, pop up scares in a scary environment.
And the movie will unlikely be any better than any other generic C/B rated movie out there, it was never "unique" it has been generic through and through and never been good or truly scary.
I can tell you the plot line right now, "Teens go out to woods for vacation/party/other, they spend the night and find things that link them to Slenderman. Generic party girl feels like she's being watched, they move on with their whatever they're doing and then they start to hear things and others thing they see something, Generic boy says he/she is crazy and then they get taken after being separated from their group." And it follows on like that until one or two of them escape the woods, their lives changed forever by Slenderman.
Welcome to Generic 101, institutionalized by Hollywood/Media since the dawn of B rated movies.