To be honest with you, I do things, as I go. I have general idea for the main story, that means story around MC's family. Anything else is created right at the moment, when I add it to the game.
Size of the game and stability of the engine might be problem in the future, for sure. I will expand the game until the it becomes unplayable because of the crashes. I create update packs, so players don't have to download the whole game every time, so that should lower the frustration of downloading large packs everytime.
I have plan B in case the game will become too unstable. I'm not sure if it will work and I will try it, when we get there. But we're far from that point, so don't worry.