Nargott
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No matter how you try, you won't be able to describe the factions' worldviews in the same way as in Heroes of Might and Magic VI and VII because there are only 4 schools, not 6-7. It's like trying to put pants on a horse; you can take two pairs and cut a hole for the tail, but... there will be serious inconsistencies, and in the case of fire regeneration spells, there will even be discrepancies compared to Heroes of Might and Magic VI and VII. At least these inconsistencies need to be removed.
That is, if you believe that the old schools are incompatible with the modern Ashan, and the new ones are compatible, then you need to at least show this compatibility by adequately assigning spells to the elements. First, try to assign them as canonically as possible, based on the fact that there are no restrictions, and then you can apply the existing restrictions, trying to achieve minimal distortions. I'm not an expert on Heroes of Might and Magic VI and VII, so I can't give you more specific advice.
That is, if you believe that the old schools are incompatible with the modern Ashan, and the new ones are compatible, then you need to at least show this compatibility by adequately assigning spells to the elements. First, try to assign them as canonically as possible, based on the fact that there are no restrictions, and then you can apply the existing restrictions, trying to achieve minimal distortions. I'm not an expert on Heroes of Might and Magic VI and VII, so I can't give you more specific advice.