I'm sure like Isla Nublar, you will still have to unlock everything for it to be accessible in sandbox. And just like Nublar, you won't be able to unlock it in sandbox.
In short, sandbox mode is what Nublar is, except now the settings will be available on every island.
We have to wait till tomorrow to see how, if it is, tied to the campaign mode.
But yes, sanbox will allow you to do whatever you want, with exclusive rules to sandbox mode that allows you to adjust the weather, cash, dinosaurs, and much more