The MMO terminology for "remain within a single game world" is single shard. EVE online is the only major MMO to attempt stuffing every player into a single shard.
Lucky for you they published a very informative article on how they do it.
The bad news. You cannot apply EVE online's techniques generally. Their solutions are absolutely tailored to their particular genre and implementation.
NOTE: For all of EVE online's super fancy single shard network they use one database. They were unable to design a scalable, consistent, moderately real-time solution for distributed databases.
Either way reading how they did it ought to help you design you ownr solution. Beware however, you are attempting to solve a very difficult problem.
Instead of distributing your game server I would suggest explore your other avenues first.
- Profile your game server.
- Optimize your server code to keep CPU burden down if that is a problem.
- Optimize the communication protocol between the clients and the server to cut down on network chatter.
- Optimize the gamer server to database communications.
- run a query optimizer then make changes as appropriate.
- cut down DB interaction to a bare minimum
- Move the DB to a separate machine.
This often helps a ton. Keep the DB on the same local network if possible but that ought to help your game server be a lot more peppy when it's the only thing running on the server hardware.