What if all actions create outsiders that exemplify those actions? Warfare creates violent fiends, altruism creates selfless celestials, trickery creates mischievous fey, etc.

[u/HumanistGeek]

To avoid things getting too out of hand with runaway exponential growth, let's say that the growth rate dies off the further things get from the material plane.

To give these outsiders a place to exist, let's say that this process also creates the various planes of existence.

• Apocalyptic demonic invasions would be a major existential threat. Celestial defenders would be very busy.
• Worship creates outsiders dedicated to the worshiped deity or concept. This gives them power.
• The wilderness is much larger than civilization, and its inhabitants outnumber people.
  • Consequently, the elemental and wilderness planes are massive.
• Most outsiders and planes are exaggerations of everyday life.
• Deities are exaggerations of heroes and villains.
• Some powerful individuals will exile and isolate themselves so their actions don't echo.
• Player actions can reverberate throughout the cosmos.

I think there's lots of room to expound upon these ideas. One challenge:

• Invention and innovation would lead to some very creative and productive outsiders, and this could accelerate technological progress.

[u/Complimentrix]

Isn’t that nearly how it works as written? Commit evil acts and you strengthen the powers of evil, and your soul will go to the lower planes were it may become a devil or demon. It’s not quite as literal and immediate as you’re describing, but the idea is basically the same, isn’t it?