I think it'd be extremely short sighted to say Universal is running out of IPs for HHN. They will always have a back catalog of more niche horror titles that can fulfill the 'cult' spaces, there will always be new deals that can be worked out with studio partners, there will always be new horror films and series coming out that they will want at the event for synergy reasons, so on and so forth.
One of the biggest trends of recent HHN history has been how much Universal has wanted to stretch the means of IP in an effort to not only reinvent what a haunted attraction can consist of but to, obviously, expand the demographics the event attracts. Working with properties like The Weeknd, WWE, Stranger Things, Five Nights at Freddy's, Fallout, etc. have proven they have every intention of making the event bigger and bigger as much as they can and reaching any and all horror adjacent avenues of potential newcomers. And I think that's a good thing!