I'm going to reply and reiterate a very old maxim - premature optimisation is the root of all evil.
Your goal *may* be to make the "open world RPG" with 2000 NPCs and a zillion zones, etc,
But you might be overestimating the amount of content that you can reasonably create. You might subsequently change the scope of the game to be a bit ... smaller.
So don't do anything. Make a few NPCs work in a small world, and once you're happy with that, then you can expand it as you have additional content.
After all, a game with 2000 NPCs and a zillion zones, is really BORING if you don't have varied, detailed content to go with it.