One of the best things about epic/high level play (in any system) is the ability to become one of the movers and shakers of their world. The character’s decisions begin to have a visible and persistent effect on the world around them. The rest of the world starts to look to them for leadership (or fear if they are chaotic/evil enough). The characters become heads of wizard’s circles, kings of nations, high-priests of religious sects and leaders of thieves’ guilds. Even if they make no choices in regards to ruling an organization, they still become the greatest mage, fiercest fighter, most pious cleric and craftiest rogue the world has ever seen.
But now the question is, how to judge when a character can be considered the “best in the world”. Is vanquishing the ancient, evil dragon enough? Defeating a god? Those are singular achievements wherein the characters can feel good about themselves. However, true pre-eminence comes from acknowledgment by their peers. Characters become the best in the world when everyone else says they are.
And this is where organizations come in. If there are organizations within the world that are touted as the being the best in the world, then player character involvement with these organizations will highlight their place in the pecking order of the world.
Or that’s the theory. The mage can not become the head of the nation’s Wizard’s Circle unless there is a Wizard’s Circle to become the head of. Likewise a nation to rule, religious order to head or thieves’ guild to run. In order for a character to become the head of such an organization there first has to be such an organization in existence. Thus it is a good idea to create these organizations at the start of a campaign.
Take a look at your player characters and do some guesswork as to what they would like to aspire to. And then place such an organization into your world. Even if they end up deciding not to head up the organization you had planned for them, the organization is still of use. It can be used as a catalyst for showing everyone the epicness of the player characters by providing adversaries for them to vanquish.
As an example, if you have a mage in your group you could create an organization called the Arcanum. This group would be known as a society made up of the greatest mages in the world. In the early part of the character’s career he may simply aspire to join the group. Once that is accomplished he can start to move his way up the ranks until he becomes its head after some great deed. It would be a sign to the players and to the in-character world that the character is now an important person in the world.
And what if the player has no interest in the Arcanum? Then, as the player character starts to reach the epic levels, he will find himself at odds with the Arcanum and they will end up sending their best to challenge the player character. And after the character defeats the best the Arcanum sent against him, he can similarly declare himself the best mage of the world.
While organizations are the best target for a character to aspire to, they are not required. Sometimes NPC individuals can work the same. If you have set up a single NPC as the best in the world then defeating that person can be enough. A couple examples of this are Elminster from Forgotten Realms or the Dragon from Dark Sun.
The point here is to set up something as the “best in the world” so that when the players defeat/replace them it is a clear sign that they are now the “best in the world”. Player characters can not aspire to greatness unless there is something to show they have accomplished this task. And to do this they need something to aspire to.