It really comes down to marketing. As an out of state fan, I can tell you everybody I talk to about HHN has NO idea what i'm talking about except for one or two people that love Universal but have never done HHN. If your not looking up theme parks on google, odds are you will never see the ads for them. They never show HHN ads on TV in Texas. I feel like for most out of state people, you have to either hear about the event from somebody that went or at least have an interest in theme parks and look into them from time to time. I'm honestly surprised Netflix isn't promoting it on their website. That would be a quick way to reach a REALLY large group of people all at once.
While i'm sure Stranger Things will increase the crowds, I don't see it being a huge increase. In fact, I think we might see overall shorter queue times for most houses considering we are getting one extra house and will have one extra ride open.