Your next park will be available as soon as you complete the requirements that appear in the tab "Parks." Go to the upper-right icons (tap on the one furthest to the left) to open the menu. There, you can check which rides are available to build ("Levels" tab) and the actions you need to complete to open a new park ("Parks" tab). 

Now, the new gameplay mode is linear, and you can't visit the parks already completed anymore once you accept to travel to the next one.

If you have completed all the parks available, please notice that the team is continuously working on new theme parks with exclusive, new rides! 

Stay tuned!