With respect to others, extreme Individualism vs Collectivism reminds me too much of today's political debates.
Here are some additionals:
The Value of Karma: Due to a bad agreement, all souls are doomed for hell, or heaven is just picky. In this system, there really is a weights and measures system. Heroism is one method.
Mixed Mythology: Heaven and Hell are a mythology crafted by the human dreamstate. Therefore, you have multiple Heavens and multiple Hells. The power of the mortal is really the power of its dreaming state and imagination, which causes such things to be real and is one of the reasons that all forms of Hell quest for the mortal soul. This does not mean that good or evil do not exist; only that while the core of them may be similar, the particulars vary from place to place, and the actual, living mythologies enforce it.
Right to Destiny: A different take on the above could be that Hell was the original creators, but the mortals, once slaves, were able to dream themselves free. This is the true value of the soul: to give dreams energy and power, over time. (Because this happens over time, how or if you have this mechanic fit into the game world is up to the DM).
It was then that mortals created Heaven. Heaven was the wish of all their dreams and wants. It is perhaps: A pure realm that Hell fights against their achieving by draining their souls and in the meantime, they're stuck in this middle, or only partly-achieved state. Alternately, perhaps heaven is too pure of a realm, and mortals find themselves doubly trapped. Even more alternately, heaven-as-made-by-mortals is really mixed up (some people are just twisted, and this was created by the shared dreams of -everyone-). It can save them from Hell, but is it worth it?
Heaven as Pure Ideas, Hell as Impure Fragments: In this, Heaven represents the purest form of creation, the purest forms of philosophy, of ideals, concepts, and ideas. The plane of fire might as well be one of Heaven's many parts. Most of Heaven is unviewable by mortal eyes, and is incomprehensible to the uncleansed body. Hell becomes the unworkable, castoff fragments of these ideals, who over time, go mad on their own. It is merely a consequence of creation and nothing is to be done for it. Think of it as the leftover parts of a sculpture, as broken clocks, untruths, as a shattered landscape and the fallen buildings of an architectual failure.
The war of mortals then is to protect their own plane of existence. While imperfect, the Mortal Realm is comprehensible to them, it is livable and new creations (from Heaven, this place of pure concept, etc.) filter downwards and generally do not harm them.
However, the leftovers do, these bits of bitter, useless leftover creation who did not "make the cut." Mortals train themselves to combat these fragments. The nature of the Mortal Realm, with its position between the Pure and the Impure, makes it the perfect battleground.
Faithwise, this world could be very practical with "what is good" being focused on local customs or survival...or it could worship ideals, which Heaven embodies. Good would be honoring what is Ideal and rebuking what is Corrupt of that Ideal. For example, an engineer would strive to embody and honor true design and strong safety measures, while rebuking what caused buildings to crumble. An engineer warped by Hell would infuse buildings with flaws, or build bridges to fall.
An artist could be both a savior and a sinner. On the one hand, an artist might use art as war, reclaiming fallen structures and giving them purpose. On the other, a deconstructivist would stand for exactly the opposite: tearing down what was once Ideal and rending it to rubble.