I have to say, there's a nice sized chunk of land behind Thunder Falls, but it's not THAT huge. It would be very challenging to fit a large-scale indoor/outdoor attraction there with a queue, gift shop, etc.
If I ruled the world I would:
1) Demolish Thunder Falls
2) Move the JP entrance gate to where the River Adventure Splashdown is
3) Have KONG be it's own new "island" between Toon and JP. The aesthetic for both Skull Island and JP are similar, so I'm fine with the occasional P-Flyer overlooking Skull Island.