Another sandbox idea

My imagination has just been flying away with me this weekend. I have conceptualized another sandbox world the basis would be this. Essentially powerful beings showed other beings the power within themselves known as magic. Through magic realities could be crushed and anything perceivable could be accomplished though reality can only be bent so much. Mighty empires where developed totally non dependent upon technology and a great civilization arose. Time passed and the power to conceive broke down tearing the fabric of reality and creating a world that does not exist in any reality except the smallest faction. Those who rose out of the ashes made a new city encapsulated within the fabric of reality that was left standing. A great city within the bounds of reality when outside monsters, horrors, and the left overs of past ruins are left standing filled with treasures from the past for adventurers brave enough to venture out.