The most powerful known wizards in Wyre and its dependencies at the end of the Seventh Century were, in no particular order of precedence, Jovol the Grey, Hlioth the Green Witch, Waide of Hethio, Mostin the Metagnostic, Shomei the Infernal and Tozinak. They were, compared to those great names of history such as Tersimion and Fillein, a group of only moderate power. Nonetheless, they commanded considerable resources and, had they so chosen, could have exercised great influence in the temporal affairs of Wyre.
Sepulchrave’s Wyre stories have a lot to tell us about D&D worldbuilding. One nice thing about reaching high level in his campaign is that you are officially one of the foremost adventurers in the world.
I like the idea that, by around level 17 or so, a character might be familiar with the handful of people more skilled at his or her class. (In 1st edition, this was codified into some classes already, like the monk and druid: there was only one level 17 monk, the Grand Master of Flowers.)
Even in 4e, where character level goes up to 30, I’ve assumed that the world’s top practitioners of each class might be around level 19. The epic tier is saved for legendary historical figures and for PCs carving out their own legendary history. There’s currently no living, epic NPC in my campaign world. Really, level 20 is plenty powerful enough.
When the PCs’ levels start getting into the mid-teens, you might want to think about the world’s level 18+ NPCs. (In Sepulchrave’s example, there are only 6 wizards of the highest level. That seems like a good number to me.) As the PCs become powerful, the NPCs will become aware of them, as possible allies or rivals, and possibly make discreet (or violent) visits.
As you construct your adventures, don’t feel constrained by your list of top-level NPCs. I bet that besides the six “most powerful known wizards in Wyre”, there are as many unknown masters. This might be even more true in secretive classes like assassins and thieves.
I also like the fact that Sepuchrave’s world contains more-powerful (presumably epic) wizards from the past. Coming up with some of these might color your campaign world as well.