My bet is that they will use a small portion of the new land for a year-round HHN-style park with permanent walk-through houses and a few horror ride-based attractions.
It will be open nightly, say, from 4pm-10pm. Houses can open/close based on season/attendance. This would be hard to resist for people after a day with co-workers at the convention center, or folks that just chilled by their pool all day, or at an off-property theme park. It's a bite-sized commitment, so to speak.
This is what Universal can do that Disney can't, and it's something Universal does very well. I'm sure the UC folks are dying to design permanent haunts, using everything they've learned over the past couple decades.
After this is up and running, we will laugh at the days when HHN had to be retrofitted into a park that wasn't built for it.
I know there are many reasons why this isn't a good idea -- no reason for you to list them -- but alas, this is my bet.