I see a lot of interest as well as debate on this subject. To put it simply, Chernarus can NOT be expanded beyond its borders.
Chernarus is constantly being expanded within its framework with additional towns, reshaping of landscapes and etc.
Below is Dean Hall on reddit answering a few quesitons as to why they can't expand beyond the borders.
When asked why the dimensions of the map should not be increased and was the decision based on a design or technical perspective, Dean had the following response:
And the last and final question in the discussion:
Link: https://community.bistudio.com/wiki/Mon ... figuration