Another concept that''s been niggling at me for some time.
For most MMOs (heck, most RPGs), gradual strength buildup is centered around the character. You gain Exp., grow in levels, and learn new abilities. Regardless of whatever items you may have with you, they're only there to enhance or supplement your own strength. In many cases, you won't even be able to use certain items without a certain amount of internal character strength.
While this does present a rather level playing field (anyone can achieve the same levels and abilities, given time), it also results in a somewhat dull and/or stagnant aspect of gameplay. Suddenly you have grinding and number crunching as part of the eternal quest to become better than everyone else.
What I'm thinking of, however, is exactly the opposite. Instead of an internal leveling system, we have an external one. Allow me to explain.
Imagine a world that has a number of special locations in it, and that has been generously littered with a number of randomly-generated artifacts of varying power. When I was first thinking about this, the idea was centered around shamans and witch doctors, and items imbued with spirits and "Juju".
The idea here is that all characters start out the same, and remain the same. They're all just human (or slightly above that, actually), and don't gain experience levels. They don't generate stat points, and they don't pull new skills out of their bums. A more level playing field you cannot get.
During the course of their travels, they will happen upon certain ingredients, artifacts, or base items that can have a spirit trapped within it to provide you with power. These items can serve as stepping stones for defeating the guardians of more powerful relics or highly concentrated spirit wells, thus allowing you to enchant/find more powerful accoutrements.
Due to the randomized nature of the artifacts and their locations, this will result in some people getting far luckier than others. I'm not going to argue that this won't cause a significant gap in power, but I do think that this system has its uses.
I do think that it should be possible to take these items away from their wielders. This means that although it's possible to nab some ultra-powerful artifact and claim yourself king of all creation, it will result in everyone else banding together in an attempt to take it from you. Tyrants and griefers will only be as powerful as the items they hold, and those can be taken from them, given enough cooperation from the oppressed. Of course, you'll then end up with some squabbling over who gets what loot, but that's their problem...
High-powered characters will be regarded as either exceptionally lucky or exceptionally devious, rather than just people who have devoted the intense amount of time necessary to attain a high level of experience through grinding.
This system does present a number of problems however... But I think some of these arise from the way we currently view items, which has been spawned from a lifetime of character-based leveling games. In such games, a character may well have a torso covering, pants, shoes, helmet, amulet, two rings on each hand, weapon, shield, and a number of other minor items scattered in as well. If we reduce the number of items but increase their importance, then we can start rethinking a new item system.
People who play the game for long periods will only be able to lord it over the newcomers through their better understanding of the game mechanics and the higher likelihood that they've managed to find some powerful items along the way. Unlike traditional RPGs, they will never be entirely safe, as they can't just continue to level at the same rate as the newcomer and thus eternally outmatch them. If the new guy gets some good Juju, then you've got yourself a fight brewing...
This results in a state of constant chaos, where the people at the top are never completely secure and the people at the bottom are never completely helpless. There will be duels, backstabbing, minor wars, and deals made for the sake of getting better equipped to handle the world around you. And, as always, the RNG will find a way to throw a wrench in the whole thing now and again. And since you'll never be able to "max out" your character, even the veterans will be constantly exploring the world in search of new treasures.
The idea needs some refining, no doubt about it... But I do think it has a certain amount of promise. I've never seen an entirely external leveling system in a multiplayer game before (unless you count FPSs or something...).