I thought I'd present a unique take on outsiders for your D&D type game. I've always been interested in the fact that some outsiders were souls of the deceased, namely angels, so why couldn't all outsiders be like that?
Birth of an Outsider
In the land of Terra, once any living creature dies, it's soul departs to grand palace of Elmawt, god of death, where it is to be judged on its life. Elmawt knows all and can read the true nature of a person just by glancing at them. If the soul had remained neutral in life, it is reincarnated back on Terra in a newborn. However, if the soul had favored one of the great Four Virtues, Elmawt transforms the ethereal creature into an outsider.
Those who remained mindful of the law and crusaded for king and country will become the stoic inevitables. Those who believed in random chance and the unexpected become the primal genies. Those who were good and kind souls, charitable to a fault become the wise angels, and those who reveled in evil and wickedness become the vile demodands. These "minor" outsiders usually serve a god of the virtue they hold dear, or as aid for a higher ranked outsider.
As an outsider slowly adapts to its form, generally it will start to have interest in another of the Four Virtues, his form will literally change into a final step or "evolution". Some outsiders do stay loyal to their original form and never change, but the number is relatively minute.
Inevitables can either embrace true law and become polyhedroids, they can favor goodness in law and become archons, or they can favor control and exploitation in law and become devils.
Angels can follow true goodness and become kheurbim, they can see the aid of control in helping others and become archons, or they can embrace freedom and love and become celestial fey.
Demodands can follow their evil ways and become daemons, they can revel in chaos and become demons, or they can lust after control and become devils.
Genies can keep chaos in their hearts and become xoac, they can turn to darkness and become devils or they can embrace good and protection and become celestial fey.
The neutral outsiders, the vecra, are an unknown force who dwell in the Veil of Darkness. They are older than time itself, and some believe they are the very essence of time. Vecra seem to be numerous and ever breeding, but souls do not become vecra.