Old mate John Longhurst was going to use the northern land beyond the creek for a shopping centre, I can't see why Ardent working with QIC to develop the completely bonkers amount of excess land will be any different.
My personal issue will be if they try to build town-houses or do something that infringes on existing land that's actually used by Dreamworld past or present. I've thrown a line on the satellite map but even without the obvious white marker, when you look online the line is practically already there:
If we're talking long term (next two decades) Corroboree will probably expand onto the island, White Water World will fill out the Thunderbolt site, Ocean Parade will be expanded - from there, the only land that's left is the Ampitheatre / Blue Lagoon site for major development into something entirely new. That's it.
If you're a regional US/Japanese park, that's incredibly limiting if you want to have a comparable attraction line-up with two or more staple coasters and large family attractions. However, if you migrated all the wildlife into a third gate a little north, then you've got a huge amount of land to play with again.
TLDR: North of the white line? Go nuts, but anything south of the white line wouldn't be ideal for long term growth without major restructuring.