In terms of Universal-owned horror properties (just going by Letterboxd's records, which may not even be entirely accurate as some properties may have changed hands along the way to 2020), there isn't a ton of IP ground remaining to tap without getting very, VERY creative:
Like, I'd love to see The Sentinel get more recognition. But it's a D-List film to even the majority of horror fans and barely on any GP radar of any generation. And there' are quite a few creative liberties that would have to be taken. These would all definitely be only reserved for the random "obscure" IP of any given year.
The Legacy and Ghost Story don't have much to offer for a house. Videodrome is a personal favorite, but I can't see it ever translating to a house for a multitude of reasons, haha. Things like Prince of Darkness and Serpent & the Rainbow kind of fall in the same boat there. The Relic would just be a monster puppet doing the scares so I'd pass on that; perhaps an original inspired by it would work better. Drag Me to Hell or Mama could be visited, but not sure if there's enough there for a full house. The Turning and Black Christmas flopped but could still get houses any year, if only as a cheap last-minute reserve.
I know Season of the Witch was skipped over from 2 to 4 (for good reason, Myers sells). But I'd be lying if I said I didn't think that ridiculous robot druid cult would make for a wicked fun house. I'd be interested where Phantasm lies right now, as Universal had a hand in the 2nd installment. That feels like one of the most notable horror franchises that hasn't been tapped at all yet. Dr. Giggles is never happening but good lord it should! The Frighteners is probably never happening but REALLY should. I could totally see Demon Knight adapted into a house just to revisit TftC (much more likely than Bordello of Blood). Virus would work perfect as a house with plenty of gore, costuming, & puppetry and should definitely be visited at some point. The rights of Snyder's DotD must be wonky, because I'm shocked that was never adapted into a house (probably conflicts with the original material's copyright, not to mention Walking Dead getting the spotlight in the modern era of HHN). The Strangers are definitely due for a revisit, even if Prey at Night tanked.