That and Comcast is way more aggressive as an owner than Vivendi was back when IoA was built.Yes, they do own them. However, they own just as many or more already proven IP's they could use to fill out another park. And Universal, thanks to the success of Harry Potter and further shown with winning over the extremely protective Nintendo, has made a name for going above and beyond to work with the IP holder down to the smallest details. Don't think for a moment they would chose something like "Mortal Engines" at this point in planning over any number of other established popular IP's regardless if they own it not.
As far as the "cult status" thing... They aren't going to build something because "hey, even if it fails, it'll become a cult classic!" No singular entity has any control over what films become cult classics. Its not something that can be forced and forseen ahead of time... Its just something that happens naturally over time. Sometimes a LONG time...
There's no chance that they'll even consider a scenario where the 3rd park could languish in the <5-6 million visitor range; they'll want to make sure that it can ramp up towards the 9-10 million mark quickly. The only way to do that is to make sure you have guaranteed IPs that can deliver that kind of launch and sustainable attendance.
Harry Potter, Nintendo, Jurassic World, Dreamworks/Illumination's bigger franchises, etc.; those can deliver them.
I'd also guess that they'd go to TimeWarner/Tolkien to see if there's a chance at Middle Earth and possibly even a DC land.
Given that there's no Six Flags in Florida; there's a decent chance that TW would be willing to license a DC Land for the 3rd park given what they must be earning off the Wizarding World.
But yeah you want guaranteed IPs; a 3rd park has to launch properly in this day and age, it's hard to imagine anything other than most of the 3rd park being "major IPs"...