Maybe prestige classes could be a form of later career background. The PC gains proficiencies in two skills and/or tools, maybe a language, equipment suitable for the role, such as a uniform, a dragon-marked sword, a messengers bag, a badge of office. A speciality suitable for the role. Traits, ideal, bonds and flaws again suitable for the role.
There should be nothing that significantly increases the characters power level, but should increase the role-playing potential.
To use my mention of the Musketeers earlier in the thread.
Skill proficiencies: Performance, Court dancing, Persuasion
Languages: add a language of your choice
Equipment: Court uniform, field uniform, tabard with queens insignia on, standard issue rapier.
Feature: Military rank. Can give and receive orders to others in the Musketeers depending on actual rank. Can always find shelter and help from current and retired musketeers.
Characteristics
Perhaps use the soldiers characteristics from the PHB page 140
This is a quick example of what I had in mind.
Entry into this could be at almost any level, and depend on circumstances, and roleplaying.