You're overestimating the size of the FFL plot, the structure is 185' x 145', and just the single ride building for monsters has a larger footprint than Forbidden Journey, which has a footprint of ~260' x 380'.
You can read more here on how the lands match up to existing areas:
It wont need to co-exist with SNW, they are their own immersive lands cut off from the hub by portals like Diagon Alley is. The over arching theme within the Hub is Astronomical (atm), not that I've ever paid any mind to big overarching themes or weird theme park canon