Here's a fun thought experiment:
A MMORPG. As original or boring as you like. Here's the catch:
The more you use your character, the less powerful she gets. Whether this is in terms of actual skill, or ability to move around, or number of turns per day - whatever the method, your character gets weaker. They also get statistically stronger, though, as they gain XP.
Why do this? Because you can swap characters with another player on the forums. The swapped character has full power - including their increases from XP.
Swapping back does you no favors: the weakened character is permanently weakened for you. You can easily make a new character, but even at full strength they aren't terribly powerful.
What kind of dynamics can you expect to arise from this kind of game?
Unless we add in some kind of strong "personality system", we can expect that players won't get very attached to individual characters. They'll basically just play the game without any regard to whose skin they are wearing. A "sell and discard" mentality.
So lets add a "feedback" system. Whoever's skin you're wearing, they become radically more powerful when they're teamed up with a character you wore out.
If you wear a mage down to the nub, to the point where he's completely useless to you, you give him to your friend and take your friend's warrior. If the two of you stay as a team, this warrior you took is hugely powerful because he's teamed up with your old mage.
Now characters have a "stickiness" to them. That mage you played isn't just a memory - his presence is important to you.
While I don't think this would cause role playing or anything like that, you can't honestly claim that MMORPGs do that as it stands, so I don't think that's a weakness.
We can expect to see a rise of guilds, just like usual. In this situation, though, guilds would have a very weird dynamic where the new people who haven't played the guild characters are the strongest and most important members. This isn't necessarily a good thing, but it's an interesting thing...
What do you think would happen? What sort of interesting potentials do you see?