Much has been made in MMORPGs regarding what, exactly, to do with high-level characters. There will always be a point when a player has done almost everything there is to do in a game. From the very beginning, even in beta, players are at work finding ways to turn a well-balanced system into a finely-tuned quick-leveling grind. This appeals to achievement players, who get to boast about how quickly they reached max level, or how many badges they earned, or stand around and act superior to all the noobs.
High-level characters inevitably complain that, even though they have wrung over 1000 hours of gameplay from a company's product, there needs to be more stuff to do!
So let's implement a Final Option: When a player reaches maximum level and has earned every accolade available then, if they so choose, they can begin a long series of missions that must be undertaken solo. These will be broken into multiple parts, and should present some unique challenges and reveal some incredible secrets. Once begun, the character cannot accept any more missions or join any groups. Make the way back just as hard as the way forward (maybe a penalty of some sort for abandoning the journey).
Upon completion, the character is retired. They will be effectively invisible to enemies but also cannot fight. They will roam as unkillable observers. They will also be members of a ruling circle comprised of all other retired characters. The circle will be given different proposals from the game developers and asked to vote. These could be simple things like whether to unveil a new minor enemy to game-changing changes like the redesign of a zone.
This would have the effect of not only allowing high-level characters a new, different level of elite, but would also make the gameworld on each server original. Imagine if one server voted to overrun a zone with a dangerous new threat while another chose instead to turn the zone into a peaceful area. With enough of these changes, you will see vastly different playing experiences. And you will begin to see the retired characters forced to take a role in consulting with lower-level players to see what changes people would like to see. Poorly-run servers will see players jumping ship, while well-governed ones will flourish and even be held up as a model for other retiree circles.
Retired characters would be governors and mentors. They would be forbidden from power-leveling players. Those who chose to forgo the Retiring quest could still power-level people, but they could no longer brag about having done everything. Imagine if members of the circle could be brought to task for their decisions. The votes would be public. If a certain number of non-retired players censure Retired Player A, then a vote could be tabled by Retired Players B and C to restrict A's influence. If the vote succeeded, then A's voting power decreases.
The idea is to provide a sort of endgame without actually stopping a player from interacting with the world. There could even be different endgame paths for characters: A circle dealing with placement and strength of enemies, a circle dealing with new zone content, a circle creating guild guidelines. Ultimately, the aim should be to create an incentive to get to the highest level. And the ability to alter the gameworld is a powerful incentive.