I just read about manes, and that sparked a few ideas. Nothing as expansive as the Steppe Elf stuff, but still ideas.



My first thought was basically "Why do only demons get these things?" Manes are basically just souls steeped in Abyssal energies for long enough that they turn into larval demons—but gods and outsiders of all alignments get souls, don't they? So what I'm thinking is a sort of "afterlife process" by which souls get converted to something more useful to the divine entities holding them:
  1. A mortal dies.
  2. The mortal's soul goes to an afterlife antechamber—purgatory, paradise, or anything in between.
  3. While in this antechamber, some fancy soul-magic stuff happens which purges the soul of undesired influence, replacing it with the plane's natural energies. CE fiends would purge the soul of lawfulness and goodness, essentially draining their kindness and orderliness; meanwhile, LG celestials would purge the soul of chaos and evil, daining their cruelty and disorder. (The undesired influence forms a sort of "soul slag" that could probably manifest as some kind of pest, e.g. celestial dogs spawning from the congealed traces of good pumped out of evil souls. Probably best to find a good place to dump that...maybe the River Styx?)
  4. Once the transition is complete, you have a creature which is a fusion of the desired components of mortal soul and the local energies—the mane. (Non-demonic versions probably wouldn't be called manes, but I don't know what they would be called, so "manes" it is.) The mane is halfway between a mortal and an outsider, a being with the substance of a celestial/fiend/whatever but the heart and power of a mortal.
  5. Manes have some uses, but they have many limitations; it takes many souls to create a powerful outsider. Manes are moved onto the first layer of [insert plane here], where they participate in some kind of task to accumulate power—ideally, something related to their intended purpose. (Soldiers would be sent to battle, workers sent to labor, the outer planes' equivalent of pencil-pushers shoved into a planar cubicle, etc.)
    Chaotic or evil manes tend to be put in direct competition, where they need to win, steal, or devour each others' energies to increase their personal power; lawful or good manes tend to be assigned tasks too great to handle individually, forcing them to cooperate more and more thoroughly until they literally become a single being. (CG and LE places would mix these two methods. LE manes would get stuck in a cutthroat bureaucracy, where they maneuver themselves into positions where they can drain each others' power or else claim the power and souls of their underlings to ascend; CG manes would compete to be the best and inspire others to lend their power or join their forces.)
  6. You have a new outsider to put somewhere as needed!




My second idea is focused on the fact that when killed, manes disperse into a cloud of demonic miasma that reforms as a mane later. (I assume more powerful demons also disperse into demonic miasma that reforms as multiple manes.) The idea of soul-stuff persisting lead to a setting idea vaguely inspired by Pact's Limbo.
Basically, imagine that everything that dies releases a soul-miasma that can reform into magical creatures. Mortals can kill these magical creatures and claim a portion of their soul-power; this is how they "level up". Simple idea, not original.
The default assumption is that there is a constant amount of soul-energy in the world, with the energy released by deaths being balanced by the energy absorbed by births. But who says this has to be the case? Maybe each birth adds a bit more, or certain types of acts can generate new soul power, or something like that.

Let's add in an adventuring-based economy. With the wonders soul power can bring (let's make all magic of note dependent on it), people want to kill the right kinds of monsters, gain soul-power, etc. This leads to the creation of adventurers' guilds that actively go out to kill monsters instead of just fighting the few that threaten civilization.
All is well. This new system brings great magical power to the world, and leads to the formation of adventuring dynasties—families started by great heroes, who now control adventuring guilds and hence the flow of magical power. Things are going well, but everyone knows that the world depends on the constant, ever-increasing cycle of magical power.
With more and more soul-power in the world, monsters more powerful than ever before are being spawned. As long as the power of adventurers and armies continues to increase, this isn't a problem...but if their power stops increasing, the monsters will catch up in power and threaten civilization itself. And the world is running out of unused power. In order to increase the power available, many guilds are exploiting the outer planes—invading heaven, hell, and everywhere in between to bargain with angels or slay fiends or finding some way to keep the power flowing.

This setting would be very different from Faerun. To start with, it wouldn't be a stretch to say that even commoners have 2-3 levels, which gives you a good excuse to have rookie adventurers start out at low or even low-mid levels. You might also let all players pick a cantrip or two, to illustrate just how high-magic the world is.
The setting itself also offers possibilities. I'd imagine this world is more urbanized than your standard medieval fantasy world—not to Ravnica levels, but still plenty of cities. The soul-miasma itself could also be an interesting element; it could be a mist which ebbs and flows across the land, spawning minor nuisances in its wake, with the risk of more powerful monsters spawning if the mist concentrates enough.
There's also plenty of conflict baked in, ranging from simple hack-and-slash ("Kill the monsters that just spawned from soul-mist over there") to political intrigue (rivalries between adventurin guilds or the dynasties which control them) to epic save-the-world stuff (either finding a way to keep the cycle going, or to exit it without destroying civilization as we know it). Lots of stuff.