A persistent universe online science fantasy with 3 different clients.

The Sim Game
The people playing with this client don't represent a person as much as they do a government. Their game is turn based. They control the laws, politics, taxes, development, etc of their world (or nation, or city). Much like Simcity.

The Strategy Game
These players control a group of some sort. They deal with large amounts of troops (or workers, etc). Much like Age of Empires or Starcraft.

The RPG Game
These players control a single character. Just like Ultima Online or Everquest.

All 3 of these games would be played out together in the same world. The Sim players make the laws and give the orders that the Strategy players enforce to shape the world of the RPG player. This makes for a game that is way more complex and realistic than any normal online game could ever be.

I am going to use the Star Wars universe for my examples here.

Sim player Bob (who represents Mos Eisly spaceport) decides to send troops to knock out a Jawa encampment.
Stategy Player Jenny (the Mos Eisly police) controls those troops against strategy player Chuck, (the Jawas).
Meanwhile RPG player Thomas was in the middle of burglarizing the Jawas' sandcrawler when the battle breaks out.

The Commander of the Death Star (simplayer) mounts an attack on the Rebel Base. Sending out several wings of fighters (strategy players), a few Imperial Officers decide to join the battle also (RPG players). The Rebel Base leader (simplayer) hears of this and sends out a Squadron of fighters (strategy player), while many rebel officers also join the battle, (RPG players).

The possibilities are limitless.

Some problems were pointed out by Kelrin with the 3 Client system mainly players dropping in and out.

This should not be an issue with the Sim Players at all because their game is turn based. My idea is that all actions they do are stored up and executed daily at the same time. (Just like in old Play By Mail Games.) (With the exception of orders to strategy players which would be sent instantly). The only way to truly defeat a sim player is to destroy them politically.

While the Strategy players troops would have to have at least some sort of AI to allow them to make some decisions on their own. In the event of a player dropping I would suggest an automatic strategic retreat of any troops that are outside of their normal territory.

The problem of RPG players dropping out is more complex. If they drop out in a non-combat situation they could simply vanish. In a combat situation I would have to suggest the same strategic retreat that uncontrolled Strategy troops would use.

There is also the question of loyalty to your superiors. There is nothing to stop the Strategy players from disobeying the orders of the Sim players. (Or even going independent altogether). Except that if you were a the Death Star commander and a squadron of your fighters went rogue, you would certainly send a couple of squadrons after them.