The attached image shows the land that Six Flags America owns (the areas within the yellow lines). According to Six Flags' filings with the SEC, SFA owns 515 acres with 300 remaining developable acres. With regards to total acres, SFA has the second most total acres of all Six Flags parks behind SFGAdv (2200 acres) and ahead of Great Escape (345 acres). With regards to remaining developable acres, SFA and SFGAdv (456 developable acres) are the only two Six Flags owned properties with >50 remaining developable acres (again, all according to Six Flags' SEC filings).
With regards to usage of remaining land, Six Flags America has strict regulations to noise levels at their property line. As such, unless they are building a show building or something else that would result in no sound increases, they cannot build to their borders and need to be cautious about putting rides that do cause a lot of noise (like coasters, especially launched coasters) too close to the property lines.
Additionally, SFA has a lot of protected wetlands that go right through the center of the property. While it's not impossible to cross or go around these areas, they do present challenges.
The best option (in my opinion) is the land to the south and west in the property (about where the blue placer is on the below map). But connecting this to the rest of the park would be challenging. It would be an ideal place for a separate gate water park (allowing them to incorporate the area of the current water park into the main park) or for some other separate gate attraction (like a shopping area or up-charge park) but I doubt that attendance at SFA is sufficient to justify such an addition.