If we assume the human soul doesn't end with the body, it might be fun to entertain the idea that it doesn't begin with the body either. Suppose it exists in some dimension or realm before it is born. What would such a pre-life be like? How would the events in such a pre-life affect/determine what happens in this life?
Part of the justification for the Hindu caste system was that the higher castes (priests, warriors) were good in previous lives. Perhaps you could implement something of the sort.
This does begin to fall apart when you go back and back, because there were less people then than now, and so there must be people who appear without a prelife. This could be used as a reason for not taking censuses in the area in question (if this is for fiction of any kind).
How about a massive, primordial, shared consciousness, gestalt entity? This is a near infinite being who exists outside of normal space-time. One of the main advantages I see of this solution is that it obviates the problem of an ever increasing human population - as a "soul" is removed from the primordial soup, the entity still exists at essentially full capacity. In this manner, the number of humans on earth can explode and we don't run out of pre-life souls. You could use this to explain "psychic" connections between humans, love.
If a soul desires to leave the gestalt being it needs to pull away from the single being - a selfish action which is driven by a selfish desire to have one's own consciousness and freewill and be self-determinate. This can be used to explain the Christian idea of "original sin". All humans that exist on earth desired to pull away from the single entity and become a selfish being.
Perhaps other souls are rejected from the single entity for different reasons - if they are drawing too much power from the single being, are poisoning it. Perhaps also the single entity is aware of what is happening in the next stage of existence and that some souls are pulling away and entering the next plane of existence as selfish beings. In response to that, the single being selects and projects other souls to foil these selfish souls. The heroes from the gestalt being - sent to earth to counter the selfish souls. In this way, two types of souls would be entering earth - selfish souls (desiring power, control for themselves) and heroic souls (selected by the single entity to counter and balance life on earth). This can be the groundwork of the never-ending battle between good and evil.
Easy question. We just need to extrapolate the data. For people who are going to heaven in afterlife has a positive slope, because present life is good and after life is better. Now extrapolate backwards, that means prelife they were in hell. Likewise people going to hell in afterlife must have been in heaven in pre life. If this is all true, then our afterlife is predetermined based on our prelife. So kickback, have a beer, and enjoy life.