I could very well imagine level 20 NPCs with 20 levels in NPC classes.

The ruler of a kingdom spanning half the known world? Level 20 Aristocrat.
A legendary blacksmith whose abilities to craft weapons and armor rival the abilities of the gods? Level 20 Commoner
The crafty spy who can lie, sneak and steal his way to any information there ever was? Level 20 Expert
The mighty shaman queen of a mighty tribe of barbarians from the glacier at the end of the world? level 20 adept
The only thing I couldn't imagine was a level 20 warrior, because warriors are just people who put some training in their martial ability, but have not enough commitment to become fighters. You don't become a a level 20 character without commitment.