Now you have, say, 20,000 people moving around on a map, countless campaigns going on simultaneously, merging and branching off.

Each person's home tab: their party (maybe limit it to 16 people), their POV of the world. Pop over to the instance tab: a firehose of everything else that's going on publicly. "oh damn those mines look dangerous and cool"

You could easily have thousands of people wandering around on adventures, bumping into each other