Well
@Legacy said a few pages back they can use marvel for a SZ, just not their characters. Dr.Doom's Alley would be a fantastic SZ, and as far as I know Doom stays pretty empty unless the park is packed so if there is a conflict with characters and the like they could possibly close that whole area down once summer crowds start to die down and do who knows what with it. I've always assumed since they've done it before they can do it again... With some caveats. I remember hearing through the grapevine that Stan Lee and Marvel was furious they had dead superheroes in Maximum Carnage, but only that and a few minor details had them really irate.
Also, Seuss doesn't need to open during an event spanned over two parks aimed at teens and adults.
Potter, Considering they use it for HHNJ I can't imagine the rights are restricted in the U.S... I'm sure how they haunt potter is a big deal and I doubt a maze or SZ would really take place... And also considering this rumored stage show they developed with Death Eaters, that also apparently takes place in Japan... Is probably being saved for when they do shoot for two parks again...
I mean really, even with ST apparently having these massive lines, can you imagine if they announced potter as a part of HHN UO? Even in an extremely limited capacity, It would do gangbusters and it would be absolutely slammed. I don't want to see a 1 park HHN with potter anything.
Also, LC is essentially dead now, If they aren't immediately gutting it, I'm sure it has space that can be used in the next year or two. You have JP,Kong area, TL, POE and LC available essentially with no limitations. Carnage building still stands.