Here's a thought: to cross from one level tier (from Heroic to Paragon, from Paragon to Epic) a character must fulfill a quest appropriate to his class and stature. So to 'open up' Paragon paths and level advancement beyond 10th, a character must do something that sets him up as a legendary figure in his profession. This quest could be tailored to a character's specific destiny, or it merely be something that would be enough to establish anyone as a legendary embodiment of the class ideal.
For example, to advance beyond 10th level a Fighter must achieve something like becoming the Champion of the Planar Arena, defeating the archtroll Lednerg in single combat or serving a tour of duty in the Invincible Legion of Mt. Celestia. The DM would make these paths known at some point as the tier limit was coming within striking distance, and the characters for their final levels in the tier could undertake the various quests needed to open the Paragon paths for one another. NPCs often never achieve these things because they lack a properly heroic band, or the will or opportunity to seek them out (though they may secretly dream of such accomplishments); those few who do have obviously earned their reputations.
The Epic quests would be similar, except more arcane and probably each containing at least one impossible-sounding clause.
For example, to advance beyond 10th level a Fighter must achieve something like becoming the Champion of the Planar Arena, defeating the archtroll Lednerg in single combat or serving a tour of duty in the Invincible Legion of Mt. Celestia. The DM would make these paths known at some point as the tier limit was coming within striking distance, and the characters for their final levels in the tier could undertake the various quests needed to open the Paragon paths for one another. NPCs often never achieve these things because they lack a properly heroic band, or the will or opportunity to seek them out (though they may secretly dream of such accomplishments); those few who do have obviously earned their reputations.
The Epic quests would be similar, except more arcane and probably each containing at least one impossible-sounding clause.