Hi! Interesting idea, I'm always happy to consider suggestions for improvements! So in this concept, after finishing the game, you could go back and continue exploring? That could be doable.
As for remakes: the game's license allows the code to be reused for non-commercial purposes. The artwork is not allowed to be used, but I would probably allow it for non-commercial projects.
If you wanted to use any existing code, the easiest way would be to implement the game's Platform class for the desired target platform https://github.com/evanbowman/skyland-beta/tree/master/source/platform. There is a folder containing the GBA implementation, and other folders containing other incomplete implementations of device/OS-specific code.
Feel free to let me know if you have any additional suggestions in the future! I will consider the postgame exploration idea and work on it when I have some time.