I think we need to ask ourselves how practical would having access to the entire islands would be for a theme park, especially one that is entirely animal exhibits. Not very, I would wager. They didn't utilize the entire island for the park in Jurassic World, and I find it a bit extreme to want to use the entire islands in Evolution, at least for the big ones like Sorna and Nublar.
We also need to keep in mind the possibility of a sandbox mode as its highly suggested to be a part of the game. Assuming its on Isla Nublar or a land mass separate from the Five Deaths, we should take into consideration what it will entail in terms of the area we may get to build our parks on, assuming there is indeed one.
I believe they will make all the islands available in sandbox mode, of course after you unlock Nublar and possibly get a 5 star rating on that island. Sorna and Nublar would be the best for sandbox. Of course if you want to play on the smaller areas I think should possible as well.