Interesting. Most people/websites out there seem to think a Gringotts themed dark ride will be part of the expansion. I wonder if they actually know that Gringotts is located in Diagon Alley, and therefore, according to you/Rowling, unlikely to ever be part of WWoHP? I had my heart set on a Gringotts ride, the showbulding could have been similar imposing scale to Hogwarts.
Is it possible that UOR might take a bit of creative licence and build a Gringotts Bank (local branch) in Hogsmeade Village, like they did with Ollivander's Wand Shop? I don't think it would work though as Gringotts is supposed to be in the Muggle World.
I don't like the idea of a Shreiking Shack. I guess it would be easy to retheme Posiedon's Fury into a Shreiking Shack Walkthrough attraction though. I think it would be too scary for young kids.
When they extend WWoHP into LC eventually, then I think Skipper Island Tours would need to come back which would take guests from PoE to JP, as the gateway from Seuss to LC would be boarded off for construction purposes. Seuss Landing therefore would become a bottleneck for fans of Dr Seuss only - until construction of WWoHP phase 2 is complete.
Only when you look at it that way, can you realise what a big undertaking the expansion would be in a logistical sense. The area of WWoHP, as you and others have quite rightly pointed out, is simply too busy and popular to have anything added to it at the moment.